You could use my checklist plugin to do this - set up a checklist that is 'student' marked and imports all activities from the course. Set it to 'automatically check-off items (cannot override)'. Hide all activities that are not assignments (within the checklist, not in the course itself), set the 'email on complete' setting in the checklist to 'teacher only'. Make sure all assignments are set to be 'completed' once they have been submitted.
With all that setup, you should get an email after the last assignment is submitted (if not, post back and I'll see if I can figure out where it is going wrong).