I've done some tutoring, (am in college) and one of the things that makes me feel pretty dumb is when I get a question from a highschool textbook that I can't answer. Sure, I've seen it before, but it involves some crazy ICE table or something I've forgotten how to do.
My advice, find out what the kid is struggling with specifically, or what chapters they are covering, and read up beforehand and do some practice problems out so you aren't stumped midway through the session. Also, wouldn't hurt to write out some problems before hand that you know the answer to, and have him/her solve them.