that is absolutely not my experience Jane. I have 7 nieces and nephews that are either at or have attended private schools and I can say with confidence that it certainly does not make them streetwise in the slightest. Several of them have now left and taken on employment, however, in my experience e they hang onto the parents shirt tails far more than state school kids and can't seem to do much for themselves at all. In other words it totally depends on thier personalities etc.
And it wasn't the same for me, either, but I was a day girl at a boarding school. The boarders all had to be responsible for their own uniforms -which may have explained why they looked so scruffy, but it wasn't done for them.
If Jeremy had hung around the farm after leaving school I'd have been more inclined to agree with you, but he had several different jobs, which I'm assuming the senior Bambers didn't help him to get, he'd been on two Antipodean trips, and he had his own house. He also organized the successful burglary of his family's business. I don't see how any of that equates with hanging onto parental shirt tails.