Class A and B were in a training facility, when a villain infiltrated.

The villain had a reverse shapeshifting quirk which could be used to turn opponents into animals.

One of your classmates (you choose who) got hit by the quirk and as a result is now back in the dorms as an animal (you choose which animal)

Aizawa: “Do not let it out of your sight.” Iida: “Sir calling them an it seems a bit dehumanising-“ Aizawa: “well they are currently not a human are they? Don’t let it out of your sight until the quirk wears off.” All Might: “Tsukauchi in the police force says the culprit’a quirk should wear off in a few hours” Mina: “so it’s like pet sitting?” Aizawa: “in more professional terms, yes technically it is pet sitting. But this ‘pet’ is wild, it has no history of being a home animal. Treat it as if it could attack at any time, right now they’re not a peer they’re a result of a villain. React accordingly”