How can we control how many pics the students posts or if they did not pay dues, how can we stop them from logging on?

Classmates can post up to 100 Profile photos. If somebody has joined your site you can't prevent them from uploading photos, but you can change their email address to a fake email address thus preventing them from logging in at all. If you do this make sure you put the Classmate's real email address in one of the alternate email address fields so you can replace it after the Classmate has paid dues. Doing this will also ensure the Classmate doesn't miss any class-wide emails you send.