Caernarfon magistrates heard that Lynsey Henderson, aged 12, was in a party of 14 pupils and a teacher from Anderson High who were being supervised by two trainee instructors as they bivouacked on May 20 last year beside the quarry near the Bryn Du Centre in Snowdonia.
The instructors, employed by Liverpool-based Mountain Ventures, had never been in charge of an overnight party before.
Lynsey woke up in the middle of the night, probably needing to go to the toilet, stepped over a low wall and fell into the quarry, the magistrates were told. She suffered a punctured lung, a ruptured spleen and broke several ribs.
She remained in hospital for two weeks and according to Alan Venables, a Health and Safety Executive inspector, would probably have died if she had not been found so quickly.
Ian Spence, headteacher of Anderson High, recalled his "sheer horror" at learning of the accident. "This traumatic incident shows you cannot ask too many questions before selecting a centre," Mr Spence said.
The school had received adequate assurances from the Welsh Tourist Board about Bryn Du Centre, he added. "We knew there was to be bivouacking, but we did not ask where it was going to take place or who would be accompanying the party. "We will not be using Mountain Ventures again," Mr Spence pledged.