Well, some might be really amazed. They could see a new side of Deku that they never knew before. His singing might show a different level of confidence or a hidden talent, so they would probably cheer him on.
Bakugo would likely be furious. He'd feel betrayed and would probably be the first to attack Deku, yelling at him about how he's thrown away everything they've worked for. Uraraka might be more empathetic. She'd try to understand what drove Deku to become a villain and hope there's a way to bring him back to the side of good.
Deku might be completely shocked at first. Since Bakugou has always been so aggressive towards him, he would never expect such a confession. He might just stand there, mouth agape, not knowing what to say.
He might cry a lot. Deku is very emotional, and being cheated on would be a very painful experience for him. He could also get angry, which is rare for him, but this situation would be so unjust that he might not be able to hold it in. He'd probably start to question his self - worth too, thinking that there was something wrong with him that made his partner cheat.
He might be initially shocked. Snape is used to students being intimidated by him, so Hermione's outburst could catch him off - guard. He may glare at her silently for a moment, trying to regain his composure.
Peter could also be very nervous. He's still a young hero and being called by the Avengers in such a public setting like in class could make him feel self - conscious. He might stutter when he tries to answer their call and fumble with his things as he gets ready to leave. He'd be thinking about all the possible things they could need him for and whether he's really up to the task while also dealing with the eyes of his classmates on him.
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.
Deku might be initially shocked. He's used to Bakugou's aggression, but an actual injury is different. He could be in pain and disbelief at first. Then, he might try to downplay it, especially if he doesn't want to cause trouble for Bakugou, because Deku is generally a very forgiving character.
Issei could be a bit nervous at first. He might worry about what his classmates will think. But if they show positive reactions, he could become more confident and start sharing more about his writing. However, if they react negatively, he may feel a bit down and might even regret that they saw it.
Deku's experiences with being bullied for not having a quirk. This would be important because it shaped Deku's character and his insecurities. Aizawa might focus on this to better understand the root of Deku's drive and his sometimes over - eager nature to prove himself. It also gives Aizawa more insight into why Deku is so desperate to be a hero despite the odds.