Susan Haley found Emma Farrar lying face down in the deep end 15 minutes after a swimming lesson at Woodlands school, Blackpool, on November 13, 2001.
"I shouted 'Oh my God she's in the water' and ran to the telephone. I panicked, I needed help, more help than I could give," she told Preston crown court.
Emma was a pupil with moderate to severe learning difficulties at Park school, Blackpool, where Mrs Haley was a special support assistant. She was left in the pool when the other children got out. Staff only realised she was missing when they discovered her unclaimed coat, the court heard.
Woodlands school swimming intructor Pauline Mills, 55, of Newton, near Kirkham in Lancashire denies manslaughter. She also denies breaching health and safety regulations, as does Keith Berry, who was head of Park school when Emma died.
Blackpool council has admitted exposing people to health and safety risks.
Alan Conrad QC, prosecuting, said Emma, from Blackpool, died because of gross negligence by Mrs Mills and because the schools operated a grossly deficient swimming policy.
The case continues.