I figured it out.
Each of my university departments has a List of Users. When the popup is opened, I send that list of users to the popup, sorted alphabetically, as a state (department_students). I also send the user that was clicked on (current_user). So then I use @eli 's great 1 Thing Plugin (1 Thing Plugin - Index Finder - Get the index number for any item in your list). When the user clicks Next Student, the first action is to use the plugin action to find the index number of the current_user state in the department_students list state. Then the next action in the workflow displays data in the popup: department_students item # result of step 1’s index# + 1. And vice versa for Previous Student -1.
Fantastic!
Thanks everyone, especially the legendary Eli.