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.
Snape might freeze for a second, his mind racing with thoughts. He might be thinking about how this changes things, if at all. Then he might respond in a gruff voice, telling Harry not to be ridiculous, but there could be a hint of hesitation in his voice. As the class goes on, he might find himself stealing glances at Harry, still trying to process what just happened.
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'.
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.
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.
In some Snarry fan fictions, Snape would be very concerned. He would immediately stop the class and rush to Harry's side. He might use his potions knowledge to try and revive Harry or figure out what's wrong. He could also be secretly relieved that he can be close to Harry without raising too many eyebrows. Snape's concern would show through his actions, even if he tried to keep his usual cold exterior. He might carry Harry to the hospital wing himself, not trusting anyone else to take proper care of him.
Some students might be shocked at first. They've always known Snape as this rather complex and often unfriendly character towards Harry in the original story. So, the idea of him being Harry's dad completely flips their perception.
Peter could also be a bit annoyed if it's not a good time. For example, if he was really focused on a test or a lesson he was interested in. But still, he'd probably answer because he knows Natasha wouldn't call without a good reason. He might give her a look that says 'this better be important' while trying to listen to what she has to say.
Yes, there are such fanfictions in the Harry Potter fan community. Some fans like to explore alternative family relationships and storylines, so they create fanfictions where Snape is Harry's father. It adds a whole new dimension to the story, often changing the dynamics between the characters and how events unfold.
Snape is likely to be initially suspicious. He has long held a rather negative view of Harry, so when he discovers Harry is good at potions, he might think Harry has some ulterior motives or has received improper help. After all, in the regular Hogwarts setting, their relationship is quite tense.
In these fanfictions, Snape might ignore Harry because of his past with Harry's father. James Potter bullied Snape when they were at school, so Snape could be projecting that hatred onto Harry. It's a simple yet powerful motivation for his behavior.
In this kind of fan fiction scenario, the relationship between Snape and Lily could have been explored in a whole new light. Maybe Snape would have been more open about his feelings earlier, and this could have led to a different sequence of events. Harry would also likely have a different personality, influenced by having Snape as a father figure. He might be more brooding or more academically driven from the start.