You don't know what happened but Yamada got the features of a wasp. Antennae, stinger, wings, and even stripes on his legs.

Some other teachers did too. Kayama got turned into a spider and Aizawa got turned into a moth. So-

You heard something and walked over and saw Denki holding a jar with a bee in it and taunting Yamada, his freaking English teacher.

Kaminari: "We caught one of your cousins."

And Yamada was freaking out on the floor as a result.