Simon Chittock, a sixth-former at Woodbridge school in Suffolk, was left disabled after landing on his back as he skied off-piste during a school trip in Austria. He now uses a wheelchair.
Mr Justice Leveson agreed that Mr Chittock, now 22 but 17 at the time of the accident, had been skiing too fast and lost control. He failed to pay attention to the topography, a warning sign and the slower skiers in front of him.
But the independent school should have withdrawn his ski pass after he and another boy twice went off-piste earlier in the trip, he said.
Teacher Andrew Jackson, who organised the trip, told the court that he had not done so "because the staff were trying to treat them as adults".
The school was 50 per cent to blame for the accident, the judge said and was in breach of its duty of care because staff failed to react appropriately to a demonstration of irresponsibility while skiing. But he accepted that Mr Jackson and his colleagues had not been "cavalier or reckless".
Woodbridge school, which charges pound;2,400 a term for day pupils, was ordered to pay two-thirds of Mr Chittock's costs, plus damages to be assessed at a later hearing.
The accident happened at the resort of Kuhtai in April 1996. Mr Chittock, now of Clapham, south London, was an intermediate skier and older than most of the boys.
Mr Jackson agreed, during the trial, that the boys' behaviour showed "sheer stupidity and (was) very dangerous".
The judge rejected the argument that merely telling the boys off was within the reasonable range of decisions that a teacher, acting in the place of a parent, should have taken.
"After all, this was a deliberate breach of instruction and, as Mr Jackson also accepted, demonstrated that the boys could not be trusted," he said.