I always get "who are you shooting for" and I usually just respond my school paper.
Thing which pisses me off though is that people are more then willing to take photos for free, especially of their kids and aren't willing to pay anything for them, even if i send a 640x480 with a watermark, they won't pony up the $3 for a 4x6 I charge, and low and behold, when I go over to their homes (teammates), I see my picture printed out in low res.
Best one I had was at a track meet last year (which I was racing in as well). A mom from another team came up and asked if I could shoot a few of her son. I said okay, and I showed her the pics afterwords. She loved them and I asked if she would want prints. She asked if i could just email them to her so they could print them out on their own printer, and being extremely reasonable, I said that would be $5/picture. She said, oh nevermind, I thought you would give them to me.
I guess the fact that a high schooler has some pro-level camera means that they can use you for free