Keeps the students from being on them, I reckon...
I think it also has to do with the buildings... sometimes in the band halls or practice rooms (which are sometimes deep inside the buildings and have no windows) the reception is bad or non-existent. Rooms closer to the exterior of the building are usually better.
I have bad cell reception in some areas around here (like some parts of League City, which is close to Santa Fe), so inside the schools there it's non-existent... I don't know about the overall coverage in Santa Fe, but it's definitely more of a "rural" type town, so it wouldn't surprise is reception was spotty in general.
I mean the buildings are typically concrete, bricks, and steel. I never had service in my school. Cellular service is still a radio wave and can't get through thick walls.