Melanie-Rose Wichmann, an autistic pupil, was shut in a small room alone because her school had not agreed how to work with her condition, according to a tribunal panel.
Staff at St John Fisher RC Primary in west London have been ordered to complete extra training since the incident, which the eight-year-old's parents say left her distressed. The school has had to apologise to Melanie-Rose and her parents.
It happened in February 2008. Melanie-Rose became agitated in class because she had not done her homework and thought she would get into trouble. Her learning support assistant used a "traffic light" system to calm her, without success, so the teacher told her to take Melanie-Rose out of class.
The assistant took her to a small medical room because the school lobby was busy. By this time, Melanie-Rose was shouting abuse at her, so she thought it was better to leave her alone and stand outside the room.
Melanie-Rose thought she had been locked in, but the assistant says she stood nearby with the door ajar. The school had been given advice on helping Melanie-Rose by Ealing's advisory specialist. Teachers say they made reasonable adjustments for her. The school also has a behaviour management programme, drawn up using advice from staff at a local special school.
But the tribunal panel, which met this month, said it was "not appropriate" for Melanie-Rose to be left alone in the medical room, although it conceded that leaving her there had calmed her down.
The panel said the learning support assistant was put in a difficult position because the school had not drawn up an individual strategy for handling the pupil's behaviour.