I don't see being in the center of the gym as that big of a problem, at least you aren't trying to cover the entire length of the court from one side of the room.
I would place all subs together facing the same direction (stacked or lined up next to each other if possible) to get even bass in all directions. If you can't fit four subs in the area in that configuration you might consider only bringing two (it's a high school dance, I can't see anybody in charge of the event wanting more bass than that anyway). Stacking one top on one sub in each corner would be easier to setup, but the bass response will be dissapointing anywhere but on the DJ riser pretty much (without phase and delay manipulation of each sub anyway).
Place one top in each corner up high to spread the sound out over the crowd and avoid comb filtering.