My friends and I went over to one of our rival High School's one night, they were playing us in the Homecoming football game that year. They (Bothell High School) had a reader board that said something to the effect of "Woodinville VS Bothell Homecoming Game, Pop Keeney Stadium" and then the date. Well we (Woodinville) put up our own message reading " Bothell Sucks Falcon Nuts" then the date of the game. It was pretty cool, even the VP liked it, although he was required to give us all the manditory 2-day suspension.
Later in the year during soccer season, our entire team proceeded to plaster our coach's car in about 4000 rounds of various colored paintballs, we covered every inch. He was cool with it (he was the kind of coach that brought popsicles to practice and we'd all lay on the field getting tan, eating popsicles and watching the girls practice), we washed it of later that night. Again we received a 2-day suspension due to a city law prohibiting any gas-powered gusn on certain properity, we served our time after the end of the season.
Do something original, that won't get anyone hurt, and be prepared to take responsability for your actions.