Harry's reaction can be complex. At first, he may show immediate resistance, arguing that he's been wrongly accused. When he realizes that arguing won't get him anywhere, he might become sullen and withdrawn. However, this sullenness is also a sign of his determination not to let them break his spirit. He'll keep thinking of ways to prove his innocence or to show that their punishment is unjust. In some fanfictions, Harry also uses these punishments as a motivation to become better at magic, so that he can one day stand up to Snape and Draco more effectively.
In some fanfictions, Snape might punish Harry by giving him extra detention tasks like scrubbing the cauldrons in the Potions classroom until they shine. This is a common form of punishment as it's within Snape's domain as the Potions master.
Snape could also punish Draco by deducting house points. Since Draco is in Slytherin, losing house points is a big blow to his pride and to the whole Slytherin house. This would make Draco the target of his housemates' ire, adding to the punishment's severity.
Harry might struggle and try to break free. He is a brave character and doesn't like being restrained, so he would likely use his physical strength and magic skills, if possible, to get out of Snape's hold.
Often, Harry reacts with anger. He's not one to take being cheated on lightly, whether it's in a relationship or in a competition. His Gryffindor pride kicks in, and he'll want to confront Draco immediately.
In some fanfictions, Snape might awkwardly try to comfort Harry. Given his own complex personality, it wouldn't be an easy or natural gesture for him. He might pat Harry's shoulder stiffly or offer some gruff words of reassurance like 'Pull yourself together, Potter.'
He could also react with suspicion. Draco might think that Harry has some ulterior motive, especially if the confession is of a more positive nature like friendship or love. He would question Harry's sincerity and might even test him in some way.
Sirius might immediately pull Harry into a tight hug. He would be very protective and try to comfort him. He might say some soothing words like 'It's all right, Harry. I'm here for you.'
However, as the story progresses, Harry may start to understand the reasons behind Snape's actions. He could become more resigned and accept the punishment as a part of his growth. He might use the experience to become more cautious in his actions and more respectful of the rules.
He might be initially shocked. Snape is not used to receiving such cute and unexpected gifts. He could stare at the teddy bear in disbelief for a moment.
Perhaps Harry just stands there, silently crying at first. He might be in shock that Snape has been so cruel. Then, he could start to think about what Snape has said or done, trying to understand if there is any truth in it. Harry is a deep thinker, and this situation might force him to re - evaluate his relationship with Snape and his own actions in Hogwarts.