An obsession with "virtual lives" - making friends and playing games online - is preventing children growing up properly, it was claimed.
John Gibson, chairman of the Independent Schools Association, said traditional outdoor play was a "primitive preparation" for adult life because boys and girls learned basic skills.
The comments come amid growing fears that over-exposure to new technology damages children's long term development.
Child psychiatrists recently claimed that teenagers were spending up to 16 hours a day playing games on-line, with "potentially severe consequences" for their education and social opportunities.
Speaking at the ISA annual conference in Manchester, Mr Gibson said that many children brought up in the 50s and 60s were given more freedom to play outside.
"I believe much of what we did then was part of a primitive preparation for adult life," he said. "Through games such as conkers, den-building or even putting the chain back on an oily bike, we practised the skills we would later find useful, and we learned to face success and disappointment in equal measure. When your life is lived through images constructed by a technical genius from Silicon Valley played on a high definition screen, I just feel it will be more difficult to experience those important rehearsals for adult life."
In recent years, concerns over "stranger danger" have led to a sharp decline in outdoor play.
A Government-backed report last year found a quarter of eight- to 10-year-olds had never played outside without an adult, while one in three parents refused to allow older children, aged eight to 15, to play outside the house or garden.
Mr Gibson, head of fee-paying Stoke College, Suffolk, said home had become a "prison" to many young people.
"Now, many of today's parents live in a shadow of fear, afraid to let their children out of their sight for too long least something terrible should befall them," he said. "When William Wordsworth wrote 150 years ago that 'shades of the prison-house begin to close upon the growing boy' I believed he was talking mainly about school.
"But today's prison-house is just as likely to be the home, a seductive, comfortable prison for boys and girls whose nimble fingers are adept at working their mobiles and computer games, but have never used them to play conkers."
Addressing the ISA, which represents around 300 independent schools, he said schools should "help children be well prepared for life."
He said schools had to abide by health and safety regulations, but insisted rules should not "cause our schools to wilt into a boring and ultimately futile educational wilderness that neither challenges or interests our children and is unable to prepare them fully for the world they will grow old in".