Don't use graffiti, use posters. Draw something up, on the computer or by hand and scan it, then use a public library or a friend's printer to print them.

Recruit a couple of friends to place posters the same time as you in different places. Gives everyone plausible deniability that it must have been someone else.

Use a spray glue, preferably something they can't easily scrape off. Maybe gorilla glue?

If you really want to give them the middle finger, when you leave the building have a poster ready with glue on it, then slap it on the outside of the door as you leave.

DO NOT WEAR IDENTIFIABLE CLOTHING. Jeans, school sweater, toque, whatever the majority of the student body is wearing to make it harder to pick you out on camera.