St Marks is a school, not an institute of higher education, there is nothing for practitioners after High School. Harry made Molly return to school while she was his apprentice. Irwin Pounder went to I believe Ohio State for his English Literature degree.
Again thinking ahead if Maggie is a Wizard it gives her a route after St Marks for training not wholly dependent upon Harry.
It always weirded me out that Hogwarts effectively ended magical education at age 18, all the Professors had no higher qualification than their own students. No Graduate, post graduate or doctoral training or qualification.
Pratchett on the other hand had Unseen University not really as a educational institution, but as a means of preventing Wizards wreaking havoc on society, by providing them with somewhere the could indulge their fancies and internal politics with the added benefits of five square meals a day and having you laundry done.
Neither is really a good model for a modern school of magic.