Maybe the Greasers, trying to mess with the Socs! Or maybe the work of some bored rich kids who have to spend the summers in the Hamptons with parents instead of smoking weed in the city with friends. Sadly, though, none of those things:
Police are also uncertain of the incendiary substance used to vandalize the field. The accompanying letter, although unavailable for release, is takes issue with the school board's business decisions according to Anderson who said "the author was expressing their concerns and dissenting opinions as to how the school board conducts business."
So! The burning of money was presumably an unironic statement, which is disappointing, though I bet that five-page letter's pretty interesting.
Oh well. At least whoever did it was rich enough to buy some substance that the cops in the Hamptons couldn't figure out. But still: nice work. Though if you'd burned a dollar sign into the East Hampton High polo field, that would've been something. Seriously.