Also if many of the parents have been to good universities and expect the teachers to have gone to those that will also have an influence on who is hired too.
Take the Habs boys list
habsboys.org.uk/info/govstaff1112.php and take say English randomly
I Wheeler, BA, University of Bristol
C R Bass, BA, University of Reading
Mrs C B Lyons, MA, Inst Of Education, London (Head of Individual Needs)
T J P Norton, BA, Royal Holloway and Bedford New College, London (i/c Drama)
Mrs D Morris-Wolfe, BA, Birmingham University
Mrs K R Pollock, MA, Edinburgh University
Mrs N M Burgess, BA, Warwick University
A O'Sullivan, BA, Trinity College, Cambridge
D W Hall, BA, Worcester College, Oxford
P P Jolley, BA, Durham University, MA, King's College London
R Amlot, MA, Mansfield College, Oxford
S Pinkus, MA, Fitzwilliam College, Cambridge, LLB College of Law, London
That is pretty much a list of universities on the whole where we would expect the parents to have gone and where the chidlren will go. If instead it were full of Middlesex Poly and nowhere good it starts not looking very good to parents.