I don't think there's a way to combine iDVD projects without some serious tinkering. An alternative approach is to open iDVD and create a new project. You or they can now drag and drop their iMovie projects onto the menu area of iDVD (generally the lower half of the frame). You'll also need to create some submenus to fit all the videos onto a menu.
If your network prevents the drag/drop idea, a different strategy is for each student to Share/Export from iMovie into a self-contained, full quality Quicktime movie. You can drag/drop these into iDVD (as described above).
John
