I think thats exactly when you DON'T want to tell 'em. I mean, if they don't know, and they flip out, would you really want to have to hang around with them for the rest of your camping trip? I think you should just tell them. I mean, its like you said, they already think that you do it, and that you're lying to them. It would be better for you to tell them than for them to find out on themselves.

Although, if you think they really were serious about the prep school bit, then maybe not...