Guilt. That is the overriding emotion of middle-class liberals who give up on the state system when their children leave primary school. I felt it as I watched my son leave for school in his smart new blazers. And shameful though this is to admit, I was as anxious to hear bad things about the local comprehensive from parents who had taken that route, as I was to hear good things from my son about his new school: it would justify my much agonised-over decision. (Though, in fact, the middle classes who are committed enough to have stuck to their principles are quite rightly loath to give ammunition to those who have broken ranks.)

Children themselves have a healthy disregard for social division up to the age of 11. If they go to a reasonably mixed primary school the only difference they perceive is size of homes, and this is innocently observed rather than judged. "Why can't we live in a flat?" my children have often whined after going to play in the council flats directly opposite the school. But at secondary level it begins to change, and can cause anxiety for children at private school who have retained strong links with old friends still in the state sector.

Miriam Gray, a textile designer, has two children at private school. Tom, now 12, went to a state primary and resents his "privileged" status. "He's always saying he wishes he was at the local - not very good - comprehensive," says Miriam. "You know - `we could have a Playstation if you weren't spending all this money on private education.'" The children, she says, are much more conscious of "the class thing" than she is. "Everyone at Tom's school goes on holiday to glamorous places, whereas we've been camping in France. But then when one of Tom's local friend'scame round he instinctively played down even that rather modest holiday because his mate had been to a caravan site in Wales."

Miriam, however, finds herself less adaptable. Her younger son, Harry, went the prep when Tom started at his private school. "It's the prep school mothers I can't stand. Ghastly, bossy PTA types harassing you to go coffee mornings. I would no more go for coffee in SW7 than I would fly to the moon."

In schools which take just a handful of children at 11, the difference in backgrounds can be enormous, and difficult to disguise. Colet Court, the prep school for St Paul's, one of London's top academic schools, has always taken nine or 10 boys each year under the assisted places scheme. The headteacher, Geoffrey Thompson, admits that "there is always a worry about these boys socially as much as anything else", but says that he has been surprised and pleased at how quickly they integrate. "Where you might get a slight problem is with limited access to extracurricular activities, but when we know there are financial problems we can help with bursary funds." Academically also, he says, these children have to work extra hard to catch up in subjects such as French and Latin, which are not taught in primary schools.

With the best will in the world, however, headteachers may not pick up on the subtle social discomfort which can arise. A St Paul's parent admits that at parents' evenings there are clear demarcations - "you get the terribly smart Kensington ladies, the more ethnic lot like me, and then huddled together in a small group and marked out by being badly dressed, the working class ones who've come in with assisted places."

However good the school is at inculcating a caring attitude in its pupils, she believes that there is inevitably teasing "and even a bit of resentment because often these boys that come in at 11 are so very bright that they get 100 per cent without even having to work for it."

But sport is the real barrier. She cites one boy, now happily settled in the senior school: "He couldn't swim, had never played rugby, never been skiing, never even been abroad. When my son went to his house he was shocked by the poverty. It was a good experience for him to see that, very sobering. But it also meant that he was embarrassed to have the other boy back to our house because of the huge difference in our circumstances."

Cherry Coombes, a lecturer, whose daughter Joanna started this week at Godolphin & Latymer in west London, admits to feeling "faintly guilty". Joanna's older brother is at the local comprehensive, and Cherry is sure she would have done as well there. "We would have liked her to carry on with the same kind of broad social mix as she had at primary school. We certainly don't want her to come out all snotty! But it was her choice entirely. She asked if she could do the exam a week before the cut-off date. Godolphin sent a list of girls going from this area and none of them are from state schools. But Joanna's perfectly capable of dealing with it all socially. She has played tennis with one of the girls and I notice she adapts her accent according to who she's with."

For Cherry, the most mortifying thing of all is the school uniform. "She was actually trying it on the other day, poncing around in it. And to think of all that time I spent fighting a uniform policy at her primary school!"n