The only other thing I can think of is to do what I suggested earlier, but on the mail server side. If you had a google apps domain set up, for instance, you could create a google ‘group’ for each student at:
You could then add each parent’s email address as an a member of that group. You would end up with the same number of groups to manage as in my earlier suggestion, but if keeping the group management outside of discourse is important then it could work. (for instance because you want to use groups to group together a set of students in a given class)