Max Palmer, from Fleetwood, Lancashire, died while "plunge pooling" at Glenridding Beck in the Lake District in May 2002.
The Health and Safety Executive reported that the main cause of the tragedy was "serious errors of judgement by the party leader in planning and leading the activity".
However, it said the benefits to pupils of properly-managed trips were immense.
Organiser Paul Ellis, a geography teacher at Fleetwood high school, was jailed for 12 months in September 2003 after pleading guilty to manslaughter.
The inquiry identified his "inappropriate actions" and "shortcomings" in the school's management "which allowed an unsuitable leader to be in charge of a party of schoolchildren in a high-hazard environment".
Max was not a pupil at Fleetwood but had gone on the trip with his mother, Patricia, who worked there as an assistant.
He jumped into a rock pool but as he surfaced it became clear he was in difficulty. Ellis and Mrs Palmer tried to rescue him but were overcome by cold. An investigation by Cumbria Police and the HSE found that the water was 8C (46F) and the stream was raging.
The HSE said Fleetwood high had failed to follow official guidelines on trips. A statement from the Palmer family on the HSE website said: "Max was a truly exceptional boy. Max left an impact on everyone who met him and through this website he will continue to do so."
Chris Keates, general secretary of the National Association of Schoolmasters Union of Women Teachers, said the report demonstrated the union's concerns about visits were valid.
Alan Whittaker, Lancashire council's cabinet member for education, said:
"School visits have great educational value and we are determined to do all we can to reduce the risk of a future tragedy."
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