Comment Pygame! (Score 1) 237
I see a lot of suggestions on how to avoid coding which seem silly to me. High schoolers that are interested in making games are probably smart enough for a little coding, and it'll do them a lot of good. It certainly doesn't even rule out other people (visual and sound design, etc) as often the design takes as much or more time than the coding.
I really like pygame, it's:
a) python
b) fairly straightforward
c) engine-less
d) cross-platform
e) free and requires only a text editor (I like komodo edit for python, it has the best python auto-complete I know of)
It's not really the thing for a 3d game, if that's your goal (which seems out of the question) you'd need an engine probably. It does a great job of 2d games, and has no inherent 'game engine', which I like, because kids will learn about a lot of the things that an engine is doing under the hood. Have some kids design sprites and levels and sound effects, have others code up games. Writing little platformers and rogue-likes is pretty straighforward.