Not to stick my nose in where it doesn't belong, but...

Might your students be better served by learning to use a more traditional version control system than Dropbox as a repository for their class projects? Mercurial and Git both have "large file" extensions/forks available that make asset management less painful, and the lessons learned w.r.t. workflow would carry forward into many other, non-game-related programming projects the students undertake in the future.