Cell signal blockers are a violation of FCC rules, so I wouldn't believe any of the myths that pass around the schools.
I think it's more in the design of the building. Even with a tower right outside the building, there's only so much steel and concrete a 1.9GHz signal can penetrate. With a lot of the schools designed for other purposes (like hurricane shelters in my state), they are usually built tough.
But then, you're not in school to use your cell phone. You're there to learn, right?