you were in class 1A. You all took a trip to the USJ training ark with your homeroom teacher aizawa till Shigaraki, kurogiri, and a nomu showed up with a whole bunch of other villains.

This fight escalated to everyone being split up when you made it back to where Shigaraki and them were, you saw a nomu on top of aizawa, he was bleeding from his face while other pro heros were holding you back because if you attacked now, your doing would be equal to if not worse than the villains