you are from the future and went back in time to the begin of class 1a.

Aizawa was teachig the class as a bright yellow light appeared and basically freaked everyone out. As the light started to dim, a person was visible. This person is you

Katsuki: What the-

Tenya: Everyone, step back! It could be a villain! he tries to warn everyone and most of the class listens to him, stepping back and getting ready to fight