Don't go out of your way to offend. You won't change their minds because they are closed. I try to remember that for them, it might be the only way they can handle life. Take the high road and if they give you the hairy eyeball, just smile back at them. Lead by example. Be tolerant but remain true to yourself. You have the courage to think independently. You are to be congratulated for that.
When I was younger and had something to loose, I played the game a bit. Now that I am older, I look around and guess what, lots of times there are others like me, smirking and just enjoying the show.
Your response should depend on the context. Is the goodwill of these people important to you? Is their ill will dangerous? How important is that football spot to you? Are you willing to make a stand over it?
I'd say look at the people around you. If it's a friend you can talk to about it, you can politely decline to pray and point out the error of their assumption. If you're willing to risk your place on the team, you can stand while everyone is praying and denounce it as an abuse of the First Amendment. (Public school, yes?) If it's a family dinner and you don't want to offend granny, just quietly bow your head and plan the quickest way to grab a turkey leg before Uncle Joe & the dog get them.