Billable collaborators are anyone that’s in a paid workspace that isn’t read-only. Because you added your student(s) to the base as a commentor (meaning they can “write” comments), you’re paying for them.
The exception to the read-only rule is forms. A free user can add one new row of data at a time through a form. You can also share those forms and read-only views with people that are not listed as base collaborators.