They have been teleported to your world.

Class 1-A was ambushed. Out in the field of their training grounds, a villain attacked. Kurogiri, the League of Villain's warp gate caused the disturbance. The swirling purple mist opened up beneath them, causing them to fall. They landed on dry leaves, shadowed by tall trees filtering out the late sunlight of the afternoon. They were immediately on high alert, as they had been trained to do. They did not have any idea where or what was going on. Suddenly, footsteps echoed on the dry leaves underfoot, and the class all turned, bracing. They stepped forward, eyes fixed on the sound of footsteps approaching. It was you.