I think it depends what your experience of school has been (for yourself or your children).
If you feel the negatives are just about having to conform, having to cope with people you don't get on with, and therefore learning skills for dealing with the kind of situations you'll encounter in later life, then I don't think that's such a bad thing, and I think this is where the "have to learn to cope" idea stems from.
On the other hand, if your experience of school has been bullying (from either children or staff) which has not been dealt with, or a soul-destroying failure to cope let alone thrive, then the best you can hope for is to survive the experience, but you are likely to come out very damaged. In that case, the child is not learning to cope at all, and would definitely be better out of the situation.
And of course, many people HE for reasons which have nothing to do with bullying or their child not coping, but make a positive choice to go about education in a different way.
We started HE in S3 (Year 10ish), not because of any problems with school, but just as a way of getting an education tailored for DSs aims and ambitions. He still mixes with groups of people around his own age, some of whom he finds easier company than others, so I don't think he is particularly insulated from "real-life"
(BTW I was thinking of carpet factories in places like India, which are a dangerous environment for children to work )