Flour. Lots of flour. Everywhere.... bathroom, gym, halls, random lockers, classrooms, offices, auditorium.... everywhere.

Get like 10 kids or more and tell em to just buy a pound or 2 of flour (it's cheap) and just make a big mess everywhere.

Trick is coordinating it.... have to do it all at once... like inbetween classes or something. Casue if one person does it, they'll be on the lookout for others doing it. Have to do it all at once.