Pretty much what Nelson said. I've found that a combination of pointing out that personal (non-school led/sponsored) prayer is allowed, and asking if they're supporting having every student be led in a prayer to Mecca each day usually at least causes a pause in thought.
However, be ready for the 'This is a Christian Country!!' response, by which they mean that their religion (so they think) is the majority, so they should be allowed to do what they like, regardless of if it is legal or not. In that event, the Treaty of Tripoli is a good point to bring up.
I love referencing the Treaty of Tripoli. Its mention usually elicits an eloquent "NUH-UH" until proof can be produced (it is situations like this that really make me appreciate the internet). When evidence of the Treaty's veracity is provided, sometimes the America-is-Jesusland-2.0 enthusiast will cite the document as irrelevant due its age. Shockingly, they never see this as contradicting their original "America was FOUNDED as Jesusland 2.0" argument.
Even aside from the blatant Constitutional violation, I am perplexed at the local Christians advocating a football team's rampage through a banner of their sacred scripture. It seems like a rather shallow deity to worship if appropriating his holy book for high school sports games is encouraged. Somehow I just don't associate Friday Night Lights with the awesome power of infinite omnipotence...
I lost it when the newscast dramatically mentioned that the team was shutout 0-34 on the day the banners were banned. OH NOEZ, GOD IS ANGRY! WE LOSE R FOOTBALL GAME! JESUS COME BAAAACK!
I quit. I'm ashamed to say that i give up arguing with those guys. You can't win. No matter what evidence I provide it doesn't matter to them. It's just "God is great" "Jesus makes everything better" I'll pray for you" When you get to heaven you'll have much to atone for" It's exhausting...talking to a wall. I give..I'm sorry guys. I failed you.
One of my fundie Facebook friends [I do have a couple] sent me an invitation to join his "Allow God in Schools" cause, which I promptly ignored. Afterall, it's only the institutionalizing of God in schools that isn't allowed, individuals are free to pray or read the bible or form religious after-school clubs, or whatever they want. I thought that the cause was a HUGE waste of time.