For your first issue, I would use a reference selector for the status. That way you can then use a microflow to show the available status for a user role. You could check for the user and then filter out any of the statuses that do not apply to that user. Retrieve the user and the UserLookupList then filter the list as need and then associate that list to the user.
That microflow then would be added to the selectable objects for the referenceSelector.
I would take a look at the https://gettingstarted.mendixcloud.com/ this is a very good source for new Mendix users. You can use that source to help solve your other two issues.
Hi Frank,
Regarding your last question. How did you try to maximize the number of students? Do you have a count microflow? And does this already work? Or are you also struggeling with that. Please post some more detail on your last question and what you already did so we can help you out.