I agree that these things tend to be relative, and a lot should depend on whether or not the kids have the clothing appropriate for the weather (not necessarily a poverty issue as someone suggested--it could well be that it doesn't make sense for *anyone* to buy snow boots or warm parkas if the normal winter includes no snow and temps in the 50's).
But 40 degrees still seems pretty warm (says me, living south of the Mason Dixon line!) to keep kids indoors during recess. My daughter's school has a cutoff of 25 degrees, although if it's dry and not windy they'll often send the kids out in colder weather, and if it's wet or really windy, they might keep them indoors. Even if they have indoor recess, they still get the run of the gym so they can burn off some steam.
Sarah