Head teacher says middle-class families enroll their children in too many after-school activities, depriving them of family time.
Parents are so worried about failing their children that they are signing them up for everything from horse riding to obscure martial arts - as well as showering them with expensive gadgets and toys, according to the chair of the Independent Schools Association (ISA).
Graham Gorton, who will also warn that constantly judging pupils in a target-driven system risks robbing them of "the very essence of childhood", believes that being denied the opportunity to "just go out and mooch about in the garden" means young people are failing to develop imaginative and inquiring minds.
Some children are so overburdened with activities that they are getting home late in the evening and struggling to get their homework done.
"It's good to be involved in activities but I think it's really important to get the balance right," he will say. "Children like their own company. They lose the capability to amuse themselves if everything is put in front of them in an organised, structured, club type of way."
Gorton will tell the opening day of the annual conference of the ISA in Bournemouth on the south coast of England: "Over recent decades we seem to have created a 'must have' culture among our young people."