Seriously, no. Run as fast as you can because trust me, even if they were paying you, the fact that it's a game that's being done by (according by your description) a bunch of inexperienced teenagers pretty much guarantees it's both doomed to fail and will destroy every single of your nerves before it goes down and sometimes even the money is not worth it.
Kinda few things can get screwed up as badly and be more time consuming when done wrong as games (animation would be another, but most teenagers don't have glorified and money filled visions about that as they tend to have with games).

So yeah, like others have mentioned, tell them this isn't working out for you, your budged and your calendar and wish them good luck on their project (but sweet jesus if they ask you to recommend some other artist don't give names).