Currently, I only use one permission set that is allocated to all student and staff Athens accounts. What do I do if I want to use the student attribute?

If you decide that you want your users to benefit from the functionality/services that the attribute will enable, you will need one permission set with the student attribute and one without. The permission set with the attribute should be allocated to all student users, and (if you modify an existing permission set) removed from all non-student users. If you are already using Athens user groups to categorize your accounts in this way, then you can use the allocate/remove options available by selecting User permissions | Via permission sets | Allocate to accounts.