When Do You Start Bringing In Students?
Basically as soon as you can. You will need students to replace your actual staff. Also depends on your starting party.
If you dont have a lightning user you would need one to research a little faster (if not research can take AGES).
At some point you also need better nature mages, to harvest specified things (specially runewood), etc.
You dont need the students per se, but you do need to convert em to staff. Sadly the game pushes you to do it. Not in the first ten or twenty days, but after that.. you will see for yourself.