A Deadly Game of Magic -- Joan Lowery Nixon
Young adult books have come a long, long way.
I was addicted to Nixon's books (and Lois Duncan's books) when I was in sixth and seventh grade. They're pre-Goosebumps, pre-Christopher Pike mysteries. (Personally, I think that they're better than Goosebumps, etc., but that's probably just because I loved them as a kid--my sister, being a former Goosebumps addict, would probably disagree).
A Deadly Game of Magic is about four squabbling high school students that get caught in a flash flood, take refuge in a spooky old house, and quickly realize that they are trapped with someone (or someTHING) else. Doo doo doo doo...
It just so happens, coincidentally, that one of the students is a budding magician--so she's able to explain away most of the magic tricks. Except, of course, for the most pressing question: Who is it that's playing the increasingly dangerous pranks?
Very silly, not particularly good, but still somehow enjoyable. (But probably only if you grew up with them). Much more funny than scary, even though I think that it was supposed to be scary.