Before 1st class (history) started, we all put on our desks books from different subjects, then when teacher asked, what class we have, we said different answers (our history teacher actually went with us to do one math equation, before starting actual history class)

2nd class (computers), we have our main teacher, so when she went to copy something, we all set fake update screen and then when she returned, we said, that the computer started to update, she fell for it, but she giggled with us as well

3rd class (Chemistry), we said, that the room is too hot and acted, like passing out, teacher giggled with us as well

I also pranked my friend with literally setting in-browser shortcut to U+K from GACKT (he was expecting RickRoll lmao)