While a dozen members of Class 1-A were fighting villains(Mineta got expelled for sneaking into the girl's locker room and Shinso took his place)but the twelve classmates were hit by two Quirks(two of them got hit by a third Quirk before the other two). One of the Quirks changed their appearance completely and the second one sent them somewhere far away(I won't say what the third one does).
When they got to the other World they also got separated into groups of two or three and are each found by groups of cats called "Clans" that offered then to join them.