The same can be said of a public sidewalk, yet the 1st holds sway there. What's the difference? In a school, the government has to protect the kids and they can't just be whisked away. On a public sidewalk, children can be taken away by their parents. In both cases someone wants to speak. In both cases, they want to do it on public property. In one case, they are prevented while in the other they are allowed.
My point has been that the 1st Amendment is not something pron producers should be able to hide behind when it comes to protecting children from their product.