i think it would be useful to have public lists of schools/class numbers that are created by users in those classes. then allow adding public assignments so anyone else in that class can just "join" the class on the site and have the list of assignments automatically shown to them. sort of like adding a public calendar to ical (btw, does your site export to ical format?)

last.fm shows you calendar events for concerts by artists you listen to. when another user adds an upcoming concert for that artist, you're automatically notified of it just by being a listener of that artist. your site would probably have these group memberships more defined, but same concept. that way you eliminate the duplication of data by 10 users in the same class entering the same assignment, and the 1 user that wasn't paying attention in class automatically gets notified of the new assignment.