A lot of people have made fanfictions with this premise before, but here is my take on it.
(Given name first, family name last, though people will still be referred to with last names formally/politely)
One more note is that I won't be using Japanese honorifics solely because I'm scared of screwing it up.
"Hmm…" the Minister of Defense said.
"We implore you sir, accept this offer. It may be the only way we will get rid of the national threat." an agent said.
Two agents stood in an open, dark room. There were no ceiling lights in the entire area, the only illumination was from a small lamp on a wooden desk.
Behind the wooden desk, in a chair, was the Minister of Defense for the Nation of Japan.
Located in the ample darkness surrounding the desk were 8 well trained guards, two on each side.
"I still don't understand something…" the minister said.
"What is it?" the other agent said.
"Why in the world does a supernatural octopus creature that can travel at Mach 20 want to teach at some junior high school?"
Both agents looked at each other. They didn't have an answer.
"W-we don't know sir, sorry! It just said that it was 'better for us' if we followed suit. Again, sorry!" one of the agents said.
"Hrmmm… What if the monster wants to take hostages? Junior high students would be perfect, and we would have to negotiate on its terms…"
"Yes, s-sir! We understand!" the other agent said.
"Hrrmm… I accept the proposal."
Both agents looked shocked. They didn't expect that answer, especially after his last statement.
"R-really sir?" they both said.
"If it is the only thing we can do to stop it, then fine. Just ensure the safety of the students. I don't want another national crisis on hand…"
"Of course!"
The minister signed the document laid in front of him.
"The entity may teach class 3-E at Kunugigaoka Junior High School. You may leave."
Both agents walked out of the room, slightly relieved.
Two security guards stood outside of the room, in the adjacent hallway.
The minister sat in his seat.
"Hrmm… something is occurring with the guards..." he said, wondering about the guards in the room.
"Now, time t-"
Click, clock
That noise could be noticed by anyone.
The Minister of Defense put his hands up.
"GUARD? WHAT A-"
"I'm not a guard, if you're wondering." a high pitched, boyish voice said.
The man near the desk knew what this meant.
"You're an assassin."
"Yep."
The voice walked out of the darkness.
It was literally a boy, no more than 160cm. He had light blue hair arranged in an odd hair style that was hard to describe. It was as if two tiny pigtails came out of the left and right, near the top.
He had a sincere, genuine smile on his face. In his hand was a handgun, pointed at the minister.
The minister was simply shocked.
"You're probably wondering how I got in." the boy said. "Look up. Large air vents. Perfect for going through."
Above were large air vents
"But w-" the adult in the room tried to say.
"How about the guards? I just knocked them unconscious while you were chatting, nothing to worry about."
The minister finally said something.
"What is a boy like you doing assassinating people? Is someone forcing you to do this?"
"No, I'm a freelancer. Anyone can hire me. I'm known as Surprise to the public."
There was no way that could be true. The Minister of Defense didn't accept that at face value.
"That can't be true! How is a child like you capable of murdering so many important people, while making it not look like a homicide? You must be under someone's control."
The boy simply continued to smile.
"Huh, who knew someone could be so chatty with their assassin." he said. "It's simple, I was hired to do this job, and so I am. That's what I always do."
"What do you want, assassin?" the minister remarked.
"Well, not really anything. That conversation was quite interesting, it gives me something to do later on."
"Next question." the adult continued. "What would you do if I press this emergency button under my desk to signal for help?"
"I'd shoot you. You have no cameras in this room for privacy reasons, so I'd frame it as a cowardly suicide. Now the question is, would you press the button?"
The Minister of Defense knew the answer to the next question. "Of course."
His hand reached for the button.
It landed onto the plastic.
Right as the mold went into its furthest position…
BANG!
The gun fired.
The hand let go of the button.
Alarms started ringing due to the button's press.
"You were under the impression that I wouldn't shoot, huh?" Surprise said. "I am your assassin, of course I'd shoot."
After leaving a faked suicide note, he started to leave the way he came from.
'Of course, you're an important member of the Japanese government. You'd do anything to catch a criminal. Even if you had to press the button.'
By the time anyone entered the room, the blue haired boy was almost out of the building.
'I've been planning this for a while. Your guards just started lunch break. To make up for that, you have no meetings at this time. The cafeteria is far away enough that I have time to escape.'
'I had to delay the conversation, lest I wanted the people who were just here to notice. Bye!'
With that, the Minister of Defense for the Nation of Japan had committed suicide.
'Kunugigaoka Junior High, class 3-E…'
"Earlier today, around noon, the Minister of Defense was found dead on his desk. Police suspect it to be a suicide." the T.V. blared.
Nagisa Shiota was just eating dinner with his mother.
"Ah, I don't think it was a suicide…" Ms. Shiota said.
"Hmm? Really?" the boy asked.
"Yes, you've heard what they're saying on the news, right?"
"Some suspect this is an assassination by the rumored assassin Surprise, known for framing their assassinations as suicides or accidents." the T.V. continued.
"See? The news says it could be from that assassin."
"Mom, a lot of people don't even think Surprise exists. They assume the similarities as coincidences."
"Sir, how do you think the Minister of Defense died?" the reporter asked.
"Well, I think it was just a suicide. That kind of job is very stressful, I couldn't do it if I tried. Plus, the idea of Surprise as a whole is ridiculous." the passerby said.
"Than-"
The T.V. shut off.
"Sigh…" Ms. Shiota said.
"What is it, Mom?" Nagisa asked.
"You are going into Kunugigaoka for this year, right?"
"Uh, yeah."
"You don't have good grades. You'll be put into the E class…"
Nagisa didn't know what to say to that.
"Sorry…"
Something Nagisa did know, however, was that he wanted to be in E class.
'My name is Nagisa Shiota. I'm an assassin referred to as Surprise. I'm entering E class for fun, for the octopus creature they were talking about.'
The boy started walking toward his room.
'Knowing the government, they'll be putting precautions in place to ensure the students won't get harmed. If I come looking as I normally do, chances are someone may notice me. No one should notice my name, though...'
'I'll disguise myself. Though, what disguise would be the most natural for my circumstances, while making it easy for me to swap between the two, if need be..'
Nagisa thought for a while.
'A simple wig wouldn't work, it's too obvious. New identity is out of the question, we already put in the papers. Maybe…'
Nagisa realized the perfect solution.
He walked downstairs.
"Mom?" he asked.
"Hmm?" she said.
"Again, I'm sorry for my grades, really I am."
"Sigh, nothing you can do about it."
This was the moment of truth.
"Umm, Mom? I think I know how to repay you." Nagisa said.
"Yeah, what would that be?"
"I'll do what you've always wanted me to do."
Nagisa took the two small hairbands off of his head. He put one of the hairbands on the back of his head, and started tying there. This caused him to have a much more girly hairstyle.
His mom realized what he was doing.
"Nagisa, are you…"
"Yeah, mom. I'll dress, act and pretend to be the daughter you've always wanted. I hope this would be enough to make you fo-"
"Of course! Nagisa, I love you!"
Nagisa's mom went to hug her daughter.
'This would be the perfect disguise. I already look like a girl, I'd just have to change my clothes and hairstyle. I can easily change my voice to be girly, and, as a bonus, my mom would completely approve of it.'
"If you're wondering about the interview, don't worry! Your gender is listed as female on all legal documents."
'I know, I know. Even the birth certificate…'
"I see, hmm…" Chairman Asano said.
It was currently interview day. Nagisa was waiting for a decision.
"Alright. Young lady, you will be accepted into class 3-E."
Nagisa was relieved. For a second, he assumed he wouldn't get in due to the look on his face.
As per his disguise, he was in the girls uniform. His body was essentially a girl's, so it wasn't hard to get the disguise on. His hair was in the shape he showed his mother, with the second hairband located in his pocket as 'a spare'. For the chest regions, using some concealed duct tape and that something he got from a clearance sale, he was able to make it look realistic.
Note: In order to make it easier to read, she will be used when Nagisa is pretending to be a girl, while he will be used when he isn't. If you're reading this, I'm sure you don't care. EndNote
"Thank you, Chairman Asano." Nagisa said, in a perfect female voice for a junior high student.
"My pleasure."
'I got into class 3-E. For the time being, once the supposed octopus comes into class, I'll sit back and watch'
"One more thing. Something may occur this year that hasn't happened before. If this occurs, understand you won't be allowed to tell other students." the chairman said.
'It's for the octopus.'
"Al-alright then…" she said, keeping a worried face on herself.
Just like that, the interview was over. It was a lot quicker than she thought it would be.
She walked out of the room, and then out of the building, until…
Pi Pi Pi Pi Pi
Nagisa had two phones, one for work and one for personal use. This was the work phone.
He went back into his natural voice.
"Yes?"
'Err, this is Surprise, right?'
"Yes."
'Just wanted to say our master refuses to pay you.'
By now, Nagisa was away from school. He went into a back alley so as to not be seen.
"Really? Who may he be?"
'The guy who asked you to kill the Minister of Defense. He refuses to pay for both assassinations.'
A smile started to form on Nagisa's face.
"Oh really? Him? Alright then, he knows what happens if he doesn't do that."
Nagisa hung up.
'Hmm, that's the first time someone did that. It's probably because he is nowhere to be seen.'
He decided that was enough for today and started to walk home.
He switched back into his girly voice, just in case.
While walking home, however, she noticed something.
A man, about 170cm, was walking down the road, on his cell phone.
'My new target. Saburo Kono, age 25. Reporter, slowly rising in the industry.'
Nagisa had to first make a visit to her house.
...closet opening noise…
...swoosh of clothes…
...stretching of hairbands…
He was in his normal look, with the tiny pigtails. Time for an assassination.
'I try to always be in my natural and preferred getup when I do the homicide itself. The disguises can be saved for infiltration.'
With that, the short boy started to follow the aspiring reporter.
'From what I've been able to gather, he's currently doing a documentary on, well, what was it again?'
"Err, excuse me!" the reporter said, coming toward Nagisa. "Do you believe in the mysterious assassin known as Surprise?"
'Oh yeah, me.'
"Well, not really. It just seems like a bunch of coincidences to me."
"Thank you, young lady."
'Do I still look like a girl…?'
'Doesn't matter, it's time for the assassination.'
Nagisa suddenly realized he brought himself to the middle of the street while following the reporter.
'This is going to be slightly harder with all these people. I shouldn't complain, I haven't done an assassination like this in a long time.'
The boy started walking near the man in the middle of a crowd.
'He won't notice me. No one ever does.'
He never did.
Nagisa, when he felt the time was right, struck.
The boy ran, nay flew in front of the reporter, stabbing his stomach in the process. Nagisa made sure to drop the knife when he was done.
The reporter didn't even know it happened, until it was too late. All he saw a blur, then pain.
The strangest thing was that no one else on the street noticed until he fell over in pain.
'He should be dead soon, I laced the knife with some neurotoxin, which should kill him fast enough.'
'The knife is on the floor. None of the passersby noticed till he fell over, using my special talents. People will think he fell onto a knife while walking, which happened to have poison on it.'
'This should be considered a weird accident, possibly homicide due to the poison. That shouldn't be a problem, however'
'I should go home, my first day in 3rd year is tomorrow.'