What's funny about the separation of church and state, is that the Constitution says that CONGRESS shall establish no law respecting religion. It does not say that a state may not pass a state religion. There may be some other law in the Constitution that prohibits the state from doing so (that I'm not aware of). But from what I've seen, the state is not restricted by the constitution in declaring a state religion. So if Texas wants to have "under God" in it's pledge, it can.
The big difference between this incident and others, is that U.S. schools do not force their kids to say the pledge, nor give them detention if they don't. If that does happen, then you'll see it on CNN and the teacher will get hounded and, possibly, fired because of it. The Christian equivalent of this would be if a Christian teacher were forcing muslim students to come to the front of class and receive communion, or make the sign of the cross...