Why expulsion is so rare at Hogwarts...at least my take on it

[Eric Oppen]

No: HPFGUIDX 164333

I think that one reason that the Marauders weren't expelled or even AFAWK 
suspended over the Prank is that Dumbledore _really, really_ didn't want 
magically-talented, partly-trained teenagers out there on their own, fair 
game for any Dark wizards who might want to take them in and complete their 
training.

I admit, D'dore doesn't do a good job at-all relating to the Slytherins (or, 
AFAIK, any house but Gryffindor) but I do think that one reason for 
Hogwarts' existence and _de facto_ monopoly on magical education in Britain 
is to at least try to prevent magically-talented children being taken in by 
people with dubious motives.  The Jesuit dictum of "give us a child until (a 
certain age) and he is ours forever" does have a lot of truth to it, 
although there are always individual exceptions.

Someone like Lucius Malfoy, to take one example, could really have stirred 
up some serious trouble if he'd had the training of a bunch of 
devotee/disciples.  Or, worse yet, a lot of Sirius Black's relatives---the 
one who wanted to legalize Muggle-hunting comes to mind here.  With all of 
Hogwarts' faults, it does have the board of governors and the MoM to answer 
to.