Emma Farrar, a pupil at Park school in Blackpool, was left in the pool at nearby Woodlands school when teaching staff took the other 10 pupils to get changed, Preston crown court was told this week.
Woodlands' swimming instructor Pauline Mills, 55, of Newton, near Kirkham in Lancashire, denies manslaughter. She also denies breaching health and safety regulations along with Keith Berry, who was head of Park school at the time of Emma's death.
Blackpool council, which employed the staff at Woodlands and Park schools, pleaded guilty to exposing people not in its employment to health and safety risks. Emma, from Blackpool, took a weekly swimming lesson at Woodlands school with 10 other children with learning difficulties.
Alan Conrad, for the prosecution, said that on November 13, 2001, Mrs Mills allowed the children to remove their armbands before the end of the lesson.
There was no physical barrier preventing them going to the deep end.
When the lesson ended no one counted the children out of the pool or checked for bodies in the water, he added. And the three support staff working with Mrs Mills had no special training or lifesaving qualifications.
Support assistant Amanda Heys, whose role was to look after an autistic boy, left the pool to get changed a few minutes before the end of the lesson.
She said no one noticed Emma was missing until they saw a spare coat in the changing room.
In cross-examination, Nicholas Braslavsky, counsel for Mrs Mills, said Park school should have been responsible for counting the children to and from the lesson.
Simon Bourne-Arton, acting for Mr Berry, said all Park school staff were aware of their health and safety responsibilities.
The case continues.