Heads are being advised to keep putting badly behaved pupils in isolation, despite a school being criticised for shutting a seven-year-old in a tiny room.
The Avenue Primary School in Leicester received a complaint from a mother last week after her adopted son was punished for misbehaviour by being placed in the school's "time out room" - a space of about 4ft by 4ft, she said, with a tiny window at the top and no handle on the inside.
Stephanie Tate, the school's head, has since apologised to the boy and his mother. She said the time out room had been used only for a few minutes at a time for pupils "displaying violent or challenging behaviour".
Use of the room has now been suspended while the school reviews the approach with Leicester council's behaviour support staff.
But legal advisers have said other schools in England should not rush to close their time out rooms.
Guidance from the Department for Children, Schools and Families has supported isolating badly behaved pupils. Ofsted has also praised some schools' time out rooms, saying they provide an effective alternative to exclusion.
Richard Bird, a legal adviser to the Association of School and College Leaders and a former head, said: "The use of time out rooms as a strategy is well established and recommended by government - headteachers should not think that time out rooms are not allowed."
He said rulings by the House of Lords meant schools could isolate a pupil from classmates indefinitely if they were supervised by a teacher and seen as a risk to other students.
Teachers should make individual judgements about whether such a punishment was reasonable, based on factors including the child's age, how they were likely to behave and how long they would be left for.
Mr Bird added: "If isolation is to be used as a punishment, it should be reasonable and publicised to the children and their parents."
A DCSF spokesman said it was up to schools to decide whether their time out room was an appropriate size. Guidance published for schools this year suggests the room should not be overcrowded, and that pupils have reasonable access to toilets and drinking water.