I mean I haven't read anything into this so this is the first I've heard about it, but as far as I know Oxbridge contextualise their offers and rarley offer lots of places to one college, so I doubt they would've done that to this school, if they kids weren't smart enough to get it.
Oxbridge has alot of applicants and it's not uncommon that smart people get turned away in place of less smart pupils, but they don't have much to go off - grades, a test and an interview.
also I don't see why it's wrong for a school to prepare you. I dont even disagree with private schools doing it. so long as they properly contextualise it, all your doing is increasing your own odds