Ask HN: Unique usernames in educational software

I'm working on a web based educational tool (university level, non-IT students) and I'm wondering how others have handled this.

Students belong to a group, which roughly equates to a class, and I'd like to keep usernames unique in each group, but allow duplicates with other users in other groups. ie group+user is unique.

Questions: 1. Is this a stupid idea? Should I just stick to the standard 'you have to think of a username no one else has thought of'?

2. How would you implement the logon screen - Three boxes (username, group and password) or combine group and username (eg group.username)?