I use a combination of permission sets for our Athens accounts. If I want to use attributes, which permission set should I give the student attribute to?

Any attributes displayed for a user account will be aggregated from all the permission sets it has been allocated. For example, if a user account has three permission sets allocated to it, and only one of these has the attribute, then the account will have the attribute displayed. It will probably be a local decision on which permission set the attribute should be given to, although the Eduserv Athens Service Desk team will be happy to discuss your options with you.