Quint I think it is more important to understand that you get these pupils who for whatever reason are intent on setting themselves up as an attention seeking "alpha" group by establishing exclusive norms of behaviour, decadent, risk taking, pushing the boundaries, call it what you will at every school. The cool kids network across school and private / state boundaries and go to the same parties, binge drink in the same parks, experiment with drugs, wind up in the same clinics for rehab, eating disorders etc.and broadcast it all on Facebook. Snobbery is an element, some may differentiate themselves as "sloaney"or "chelsea set" but the basic behaviour and values are the same. They are in every state school, including Catholic Schools (where the nature of the boundaries may make them even more attractive to kick against, and parents may have sent pupils whose behaviour or susceptibility to influence was already a worry) and every private school.
Just as importantly they do not represent the majority. So unless your son is an attention seeking risk taker or will do anything to be cool he isn't going to want to get involved and there will be plenty of other friendship groups with values he is more comfortable with to be part of. If he is, then unlike some of the parents of these cool kids, you need to put in place some very firm boundaries. In fact as parents we all need to take responsibility, be aware of what goes on, talk to our children about it and put in place firm boundaries because otherwise we leave our children open to real risks.They are still learning but one small mistake now will be broadcast across London and possibly remain on the internet for the rest of their lives. When gossiping about one incident involving a 13 year old with my daughters recently my older daughter pointed out that with friends now emerging into the job market she is now hearing of employers routinely checking the internet and finding this sort of thing.
All these fee paying schools will support you in this. They have firm boundaries and an ethos of making sure their pupils appreciate their priviledge, and the responsibilities that go with it, of community involvement, bursaries (28 full bursaries and 128 being given financial help at LEH, ScaredySquirrel) and will lay on talks etc to make sure that parents understand the demands of modern parenting in a decadent part of London too. Secretly I know many teachers, state and private, dispair of the naivity / unwillingness or inability of some parents to set boundaries and the risks they expose their children too.