Paul Revere Rides Again - S2.E21

A mysterious masked vigilante starts appearing on the rooftops of the school, showering people with (dry) garbage and leaving anti-litter messages signed ""Paul Revere"". The teachers, or a teacher, figures out the vigilante's identity -- he's the only kid with an anti-litter bug and a Paul Revere fetish, and subtly encourages the kid to pursue his cause along more conventional lines. ""Revere"" makes one final appearance on the rooftops, sending down a paper airplane with a poem written on it. The poem ends: ""So don't litter my children, or you will hear of the return of Paul Revere.""