I went to a London private school ... ooh, 25 years ago and there was a fair amount of decadence in the sixth form at least - drinking, drugs, sex etc.
But my friends who went to comps in the middle of nowhere seem to have had not dissimilar experiences.
I think a lot depends on the child's personality, I was (and am) rebellious by nature and was drawn to "wilder" activities. However, the wilder bunch were a smallish crowd and many of my peers were eminently sensible in comparison.
all survived and done well and turned into boring grounded adults. DD1 is at a private prep school and I think so long as I hammer messages home about drugs/drink etc, I think she will be fine.
I am more concerned about the "bling side of things, that my dd (who was previously at a state primary) is now seeing everyone's parents driving 4x4s and holidaying in the Maldives. But a lot of other parents share those concerns too, so you have to hope the message that our children are extremely privileged will prevail. There were plenty of rich kids at my old school but they weren't necessarily the "cool" ones, other qualities were valued much more. I suspect it's the same today.