It's pretty nitpicky, but are there really no mermen? It seems unlikely that only girls would be desperate enough to try this...
Maybe there's an alternate equivalent spell for guys. The incantation specifically summoned mermaids.
It's possible the spell causes a Gender Bender for males that use it and fail.
Or maybe boys simply weren't desperate enough to try some voodoo hax instead of simply trying to be with girls normally.
If the mermaids are capable of floating and passing through walls, why don't they just hide on the exterior of the school, along the upper floors, away from any windows? It would be hard for anyone to notice them, and even if they did, they would probably have to climb along the exterior and risk their own deaths to get at the mermaids.