Following the events on Halloween, the entire school was speculating who could be the heir. Every Slytherin, except for Harry, was a suspect. After all, the Boy Who Lived, who defeated Voldemort, couldn't be evil. A lot of people suspected Draco.
Hermione was one of them, and she was growing obsessed, understandably. She would repeatedly ask Harry if Draco said something or did something suspicious, to which he always answered negatively.
"He wasn't there the night of the attack," said Hermione.
"I wasn't there either!" Harry reminded her.
"Well, are you the heir of Slytherin?"
"Of course not!" Harry answered, annoyed.
Harry was annoyed that no one thought it could be him. He obviously didn't want them to, but all the arguments that people used to justify believing that Draco is the heir apply to him as well!
Slytherin? Check!
Disappeared the night of the attack? Check!
He's not a pureblood, but he's not a muggle-born either.
On the other hand, he was also annoyed at how calm and indifferent Draco had been.
"So, you're not worried at all?" Harry would often ask him.
"Why would I be?"
"Because someone could die!"
Draco shrugged.
"People think you're the heir!" Harry tried a different approach.
"Well, I'm not." Draco said, starting to get annoyed by how much Harry did care. "Plus, no additional attacks have happened since Halloween. Maybe it was just a prank?"
That did not last long though. A few days later, another attack happened. This time, it was a student. Creevy from Gryffindor was found petrified almost a week after the first attack. This changes things. Students are harmed.
That prompted the school to arrange a dueling club led by no other than professor Lockhart, who was, for some reason, obsessed with Harry. This is how Harry ended up in the dueling arena against Justin Finch-Fletchley from Hufflepuff.
Harry never interacted with Justin before then. He had seen him around, shared a few classes, but they never talked to one another. The spells used by both of them were harmless, and they only aimed for the torso, per safety instructions.
"A snake!" came out of Justin's wand, making its way towards Harry.
"Don't panic!" announced Lockhart as he aimed his wand at the snake and shot a spell that made the snake fly up in the air, right before falling back unharmed. This seemed to only anger the snake.
"Stop!" Harry said to the snake. Everybody stopped to listen. The snake did too.
The snake hissed back, and then was evaporated by Draco, who was standing close by. Harry looked at Draco who seemed terrified. Not by the snake, by him.
"What the heck, Potter!" Justin Finch-Fletchley seemed terrified too.
"You shot the snake!" Harry objected.
"You talked to it!" the crowd murmured in agreement. Harry realized what that meant. He's no longer off the suspect list.
It did not help that the next day Justin Finch-Fletchley was found petrified the next day along with Nearly Headless Nick.