A/N: This is based on my headcanon that Aizawa sometimes lets Naomasa or Tensei substitutes his homeroom class.
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Izuku arrived at class a few minutes before the bell rang. He was met with a pleasant surprise when he opened the classroom door.
"Detective?!" Izuku gasped.
Naomasa tilted his fedora before nodding.
Izuku couldn't help but giggled when he saw the rest of his classmates caught off guard by the officer sitting in Aizawa's chair.
After the bell rang, Naomasa formally introduced himself to the class.
"Good Morning Class, I'm Detective Tsukauchi but you can just call me Mister Tsukauchi or Mister T for short." Naomasa said as he wrote his name on the chalkboard.
Momo raised her hand and asked, "Where is Aizawa-Sensei?"
Naomasa sighed.
"I'm not really sure. It's classified ." Naomasa said.
The Class gasped. They wondered if Aizawa was on a top-secret mission for the Hero Commission. Others figured he was probably an international spy like James Bond. Although there was fuel who thought Aizawa was some sort of vampire, zombie, mummy or some type of undead creature. Then there was Denki, who thought Aizawa was all of the above.
In reality, Aizawa was at a Cat Convention.
Naomasa flipped through the pages of Aizawa's lesson plan. He was quickly overwhelmed by Aizawa's instructions. He figured he would just go easy on them since he's not there.
Step 2: Take Attendance.
Naomasa quickly counted the number of students in the class. They were all here which made his job easier.
"Can someone take down this attendance sheet to the main office?" Naomasa asked.
Several of the students rose their hands.
"Ooh! Ohh! Me! Me!" Mina shouted.
"A-Ashido?" Naomasa questioned.
Mina nodded and got up to get the attendance sheet.
"What? I'm faster!" Tenya said.
Naomasa shook his head.
"No, Aizawa would kill me if I let you use your quirk to run an errand." Naomasa explained.
Tenya sighed and gave in.
Naomasa looked back at Aizawa's instructions after Mina returned a few minutes later.
Step 3: Make sure the students are in their assigned seats.
Naomasa looked up at the class.
"Guys, you are not in your assigned seats." Naomasa said.
The Class whined.
"Just don't tell Aizawa." Naomasa said while winking.
Step 4: I will find out.
A chill ran down Naomasa's back.
"Nevermind, since I don't want to be found dead in a ditch somewhere, can you guys please sit in your assigned seats?" Naomasa asked.
A few moments later the students were in their assigned seats.
Step 5: The students are bound to ask what your quirk is. They are nosey.
As if on cue, Izuku raised his hand.
"How does your quirk work?" Izuku said
Naomasa raised an eyebrow for two things.
" Aizawa must be psychic like Sir Nighteye or something. " Naomasa thought.
Secondly, Izuku already knows everything about his truth quirk after bombarding him with questions. He figured Izuku wanted him to impress the class with his quirk.
"I have a lie-detecting quirk." Naomasa said.
After explaining the basics of his quirk, he asked a student if they can volunteer for a demonstration.
"Me!" Denki blurted out with his hand raised.
"Kaminari." Naomasa said.
Denki cleared his throat and told him an interesting detail about himself.
The Class giggled when Denki finished.
Truth
"Truth?!" Naomasa shrieked.
Naomasa was speechless. If what Denki said was a lie that means the opposite is the truth. He hoped Denki just thinks it's truth.
He's gonna have nightmares about this either way.
"T-That's illegal! It's more than that. It's just wrong!" Naomasa said.
"Why would he tell me that? He knows I'm a cop. " Naomasa muttered.
Step 6: Yes, I know my students need therapy. We are working on that.
Naomasa sobbed when he realized he had to put up with them for the rest of the day.
OMAKE TIME!
Omake: Naomasa looks like Daichi from Haikyuu
Mina (gasping): No wonder you look so familiar! You look just like Daichi from Haikyuu!
Naomasa: Who?
Ochako (showing an image on her phone): It's an anime character from a sports anime.
Shoto: I read the manga! Daichi even grows up to be a police officer!
Denki: Bruh, Daichi really said "Fuck volleyball and went to My Hero Academia to become a police officer and changed his name."
Izuku: Sounds like a crossover. They are both published by Shounen Jump.
Naomasa (internally screaming) :
Class (nodding): Yeah
Naomasa (drenched in sweat): Shh! Don't blow my cover.
Class: Oh shit! It's true!
Endnote: I was gonna write a part where the class gets attacked by villains but my heart wasn't into it.
I kind of want to write a series where people Aizawa knows just randomly substitutes his homeroom class.