Mr Justice Bennett overturned a verdict made in April by an inquest jury held in Neath. Instead, he ruled the 16-year-old's death had been a tragic accident.
Herve Bola, from Woodford Green, Essex, died at Sgwd y Gwladys Falls, Neath Valley, in July 2002 while in the care of youth workers.
Education unions in Wales this week welcomed the decision, made on appeal.
David Evans, secretary of the National Union of Teachers Cymru, said: "Teachers who take visits of this kind have a great responsibility. It is wrong to increase that burden on them by looking for blame when all the facts suggest an accident."
But a spokesperson representing Herve's family said his mother was distressed that a judge had overturned a decision made by a jury of "ordinary people".