And what authority does the government have on that? The only authority the government has is to not establish a religion and is specifically forbidden to suppress religious expression. Besides, how is thanking God at graduation proselytizing? How are student lead prayers proselytizing? These aren't urban legends, they are court rulings.
The authority that recognizes when you use public resources to advance religion, it creates the appearance, if not the reality, of a government-established religion.
I don't know about you, but I really, really like the fact that I can go to church on Sundays without the government having to approve of what the pastor says. In order to keep it that way, we have to be extremely vigilant in keeping the government's business in the government's hands, and the church's business in the church's hands.
What people often fail to consider when arguing against the separation of church and state is that it is a two way street: you simply cannot increase religions role in government without also increasing government's role in religion. Do you really want your government to become more involved in your church?