GameMaker has forums and capability to upload your games. Partner that together with linked DropBox accounts, and cooperative development can take place.
If this is for a classroom setting, a single DropBox account just installed on all the computers would do. You might need to pay for a little extra storage space for 30+ kids, but it could probably hold 10 small projects in comfort. You get 2GB for free, though you can unlock extra space.
Speaking of which, on the off-chance you're now motivated to signup, you can mark me as a referrer and give me (and you) an extra 500mb, by using this link.