I've done this two ways in the past -- works with either Omeka or Omeka.net. I have registered all students as users with administrator privileges. Together we have built a shared Omeka archive. I ask students to put the items they contribute into a collection under their name so that I can quickly identify their contributions. Administrative users can edit and delete all items, so students need to be careful not to harm one another's work. But, this user status means that each student can use any item from the archive in her/his exhibit. Then, there are two options on the exhibits: 1) Have the class build a single large exhibit where each student is responsible for a single section. The exhibit will have a single theme, but each student can choose from the available page layouts and create many pages. This option limits the ability to single out a student's work on the homepage because you can't feature sections of an exhibit. 2) Have each student build individual exhibits. This allows each person to select a different theme, and allows for the possibility of featuring individual exhibits.