in the dorms when bakugo wasn't there:

Mina: hey guys! Wanna do a prank to Bakugo?

Deku: umh I don't know if I should prank my bf-

The others: YES!

Mina: ok, so we have to ignore him for 24h ok?

Time skip to the next day. Everyone was completely ignoring bakugo, even deku (his bf)