Izuku would probably be terrified at first. After all, he's been kidnapped by villains. But then, his hero - like determination would kick in. He might start looking for ways to escape or at least resist their plans.
All Might would be furious. He would see it as his failure to protect Izuku. He would probably mobilize all the resources he has, both in his hero form and his connections in the hero world, to rescue Izuku. And he would be blaming himself all the while for not being there when Izuku was kidnapped.
On the other hand, it could also lead to better understanding. If Izuku snaps and then later explains his reasons, it might bring the class closer together. They could realize that he has been dealing with a lot and become more empathetic towards him. However, in the short term, it will likely disrupt the usual harmony in the class. Some students might be shocked and not know how to react, while others might take it personally and become defensive.
Some characters might be immediately terrified. They could avoid Izuku or even try to drive him away. For example, classmates who don't understand his quirk might spread rumors about him.
One reason could be stress. Izuku might be under a lot of pressure to keep up with his training and the expectations of being a hero. Class 1A's constant high - energy and sometimes chaotic behavior could push him over the edge. Another reason could be misunderstandings. If they misinterpret his actions or motives, he might snap in frustration. For example, if they think he's showing off when he's actually just trying to help.
Eventually, the class might come to understand Marinette's decision. As they struggle to do things on their own, they realize how much effort Marinette had been putting in. This could lead to them changing their ways and maybe even trying to make it up to Marinette if she decides to start helping again.
You can try searching on fanfiction websites like Archive of Our Own (AO3) or FanFiction.net. These platforms usually have a wide variety of fanfictions, and you might be able to find ones related to Class 1A watching Izuku there.
One interesting story could be where Class 1A watches Izuku's training sessions secretly and are in awe of his determination and growth. They might then start to understand him better and change their perception of him.
No, it is completely inappropriate. Classroom is a place for learning and such sexual content in a fanfiction related to a character like Izuku is not acceptable in a general moral and ethical context. It goes against the positive and educational values that the original work might represent.