schools generally dislike ex students hanging around because they are seen as usurping the instructors' authority which "must" be total and unquestioned. I was pushed out pretty fast myself after I got the PPL and stopped renting and bought my plane ... As for "messing around" with the school's existing or past customers, that is out of the question and would have to be done off the airfield. And the "post qualification" line is a tough one to stick to because let's say you meet up with 3 pilots, and one of them hasn't yet got his PPL. Do you tell him to leave the room? You would just have to hope he keeps his mouth shut. Tricky!
Is this just a fixed-wing phenomenon?
From my (admittedly limited) experience of helicopter flight schools, they all positively encourage students to mix with and learn from current PPLs. And once you have the license in your hand, far from pushing you out they do all they can to keep you coming back and renting aircraft.
There does seem to be a noticeable difference in personality type between fixed wing instructors and helicopter instructors.