Those 'privileged bastards', as you so eloquently put it, pay for their own education, they may waste their education but it is paid for by their parents/guardians; if these kids went to state schools with their attitude of not having to try because they are rich an will not need to work, then they would put a further strain on the education departments already limited spending budget. Yes these schools are cheating a little by claiming tax-breaks as charities whilst only offering limited assisted places to qualify as a charity, but they do qualify and they do relieve some of the strain on the state schools, without these schools I think the education system would be worse than it currently is; instead of more schools or more classes in the state schools they would just cram more into the current classes this having a detrimental effect on them.
In the comprehensive school I went to we had around 112 pupils per year, this meant the you were graded and were in groups learning with people of a similar ability to yourself, this in itself is not a bad thing as you will be learning pretty much at the pace relevant to yourself, I don’t think being taught at a comprehensive school gives you that much of a dis-advantage if you want to learn you can and will learn, if we had the Rich kids in the schools not trying this would also be detrimental to the others kids development.