During the battle in USJ, Tomura was hit with a quirk. He though nothing of it since it didn't do anything to him at the time. But when he got back to the bar he felt weird. And suddenly turned into a baby. Kurogiri doesn't know how to deal with babies so he warped him away. Into UA.. into class 1-A.. in front of Aizawa.

"Ah- what the hell" Aizawa gasped looking at the baby.

"Wait.. isn't that the villain we were fighting at USJ? Shigaraki?!" Iida askes. The whole class gasps which frightens Tomura and he starts crying. His little hands flailing around and his little chunky legs kicking. You can't tell if he's scared or if he just hates being here.