Annie Woodland, 24, who lived in Essex, suffered brain damage in the incident at Gloucester Park Pool in Basildon.
A teacher and a pool life guard both failed to realise she was in trouble.
The family has successfully sued Essex County Council who said it did not comment on individual cases but had noted the High Court judgement.
The tragedy happened in July 2000 when Ms Woodland was a pupil at Whitmore Primary School in Basildon.
At an earlier hearing, judges ruled a victory against the county council would risk a "chilling effect" on the willingness of schools to take pupils on educational trips.
A Supreme Court judgement in 2013 overturned this ruling and opened the way for a High Court hearing.
Mr Justice Blake has now ruled lifeguard Debbie Maxwell and swimming teacher Paula Burlinson should have noticed Ms Woodland was drowning sooner than they did.
He concluded Essex County Council was liable for their negligence although neither woman was employed directly by the council.
Ms Burlinson's failure to notice Annie in distress "fell far below the standard of care reasonably to be expected of a teacher", he said.
Ms Maxwell was also negligent as "she was not paying sufficient attention to users in the water".
The amount of compensation to Ms Woodland, who now lives in Blackpool, will be assessed at a later date.
A spokesman said: "Essex County Council notes the judgement of Mr Justice Blake following a trial on liability.
"The authority cannot comment specifically on individual cases and it would be inappropriate to comment further therefore in respect of this claim."
A woman who nearly drowned during a school swimming lesson when she was 10 years old has won a compensation battle at the High Court.