Pauline Hobbs, who taught at Ed-Start centre, Stoke-on-Trent, a pupil-referral unit for 11 to 14-year-olds, was issued with a reprimand which will stay on her record for two years.
England's General Teaching Council found her guilty of locking a pupil in a storeroom in September 2003, but the disciplinary hearing was not satisfied the move was intended as a sanction.
It concluded, though, that Mrs Hobbs made a conscious decision to leave the boy in the room which was inappropriate, whether the door had been closed by the pupil or by her.
The panel said it would have expected a teacher with Mrs Hobbs's experience to have acted differently.
Mrs Hobbs was cleared of three further allegations of locking a pupil and a colleague in a storeroom, keeping pupils in isolation six times in September 2003 and excluding a pupil contrary to unit policy.
She first appeared at a disciplinary hearing last month. At that hearing, Andrew Whitehead, a support worker at the centre, said: "The rooms were dangerous and terrible. There was rat poison on the floor and damp on the walls. I wouldn't be happy with my children working in that environment."
Philip Ryder, head of the mixed pupil-referral unit, said he was concerned about children's safety.
Mrs Hobbs denied the charge of unacceptable professional conduct.
She resigned from the unit in March 2004 after disciplinary action was threatened.