Snape himself might be initially stunned. He has a complex relationship with Harry, and this sudden declaration could stir up a whole range of emotions in him, from confusion to a strange sense of warmth he tries to hide. He may also worry about how this will affect his reputation in the school.
One possible reaction could be shock from the other students. They know Snape as a strict and often unfriendly professor, and Harry calling him 'dad' would be completely unexpected.
One possible reason could be that in this fan fiction, Harry has developed a deep respect and a father - like bond with Snape over time. Maybe Snape has been secretly mentoring Harry in potions and has shown him kindness and understanding that Harry has never really received from his own father figure. This has led Harry to subconsciously view Snape as a father figure and the moment in potions class is just when this feeling overflows and he blurts out 'dad'.
Snape might be initially shocked. His face could turn pale and his eyes widen in disbelief. But then, a small glimmer of something like pride or warmth might flash across his face for a split second before he quickly composes himself and snaps at Harry to focus on the potion.
Maybe Harry has had some secret interactions with Snape that the readers haven't known about yet. These interactions could have made Harry feel a father - like connection to Snape, and in the heat of the moment in potions class, he just blurts it out.
One reason could be Draco's arrogance. If Draco shows too much pride and disrespect in class, Snape, who values discipline, would not tolerate it. For example, if Draco mocks other students' potions attempts, Snape might punish him.
He could feel a sense of guilt. Snape has always had a role in Harry's life, sometimes as an adversary. Knowing that Harry has these scars which are a result of Voldemort's actions, and Snape being involved in some way with Voldemort in the past, he might feel responsible on some level.
Draco could be made to stand in the corner of the classroom during the entire potions class as a punishment. This would be a public humiliation for him, especially in front of his peers. It would also show Snape's no - nonsense attitude towards misbehavior.
Perhaps he was extremely tired or stressed. Harry has a lot on his plate, with Voldemort constantly being a threat and all the schoolwork and Quidditch. He might not have had enough sleep or rest, and the potions class was just the tipping point where his body gave out.
Well, Draco could be punished by Snape for being disrespectful in Potions class. Snape might order Draco to write a long essay on the history and significance of a particular potion. This would not only be a humiliation for Draco but also a chance for him to gain more knowledge about potions. And it would add an interesting dynamic to their relationship as Draco would be resentful at first but might end up learning something valuable.
The grading system in this fanfiction's potions class could be different. Instead of the usual strict letter grades, it could be more focused on improvement and creativity. So, a student who comes up with an innovative way to make a potion, even if it doesn't turn out perfectly, could still get a high mark. Also, group projects could be a major part of the class, promoting teamwork among the students which was not always emphasized in the original potions class.
Snape might be suspicious at first. He could think Harry is up to something mischievous or perhaps trying to find a weakness in his teaching. Maybe he'd keep a closer eye on Harry during class.