And there's a very good reason for that. It's called the First Amendment guarantee of religious freedom. This guarantee means that my children or grandchildren cannot be forced to participate in your religious rituals, and further guarantees that the government will not favor any religion above all others -- which a teacher or school official (i.e., government employee) leading the school, or a class, in a christian prayer would entail.
Any child that wants to pray at school may still do so under current law, and many still do (both individually and at gatherings like "Prayer at the Flagpole"). And I (as an atheist) have no problem with that at all. After all, the Constitution gives them that right. But that is not good enough for many christian fundamentalists. They seem to think that their religious rights have been violated because they can no longer force others to participate in a christian ritual like prayer (which is really an anti-constitutional view).
Not being able to force others to participate in your religious rituals is not a violation of your religious freedom. It simply gives everyone the same right -- the right to make their own decision on what religious rituals they will or will not participate. And shouldn't that be what we all want in a free country?