David Byrne, 56, was found guilty at Oxford magistrates court of dragging a boy 20 yards along a floor at an Oxfordshire school in May 2004, leaving him with two carpet burns.
Judge Brian Loosley granted the absolute discharge, saying the incident was a "moment of madness" in Mr Byrne's otherwise unblemished 33-year career.
Mr Byrne, from Abingdon, was described by the headteacher of the school, which cannot be named for legal reasons, as a "well-respected member of staff".
The court was told the pupil, who has cerebral palsy, had been carried into the school kicking and screaming by his father on several previous occasions.
On May 19, 2004, Mr Byrne had been called to take the boy, who was sitting on the floor in the main reception, to class. Unable to make him stand up, he and the deputy head carried the boy part of the way. But he had struggled loose, so Mr Byrne decided to drag him, stopping at intervals to give him a chance to stand up.
Convicting Mr Byrne, the judge said the force he used was unreasonable and the incident was "a very serious error of judgment".
Mr Byrne's wife, who was in court with his two daughters, collapsed shortly after the judgment and had to receive medical treatment.
A spokesman for Oxfordshire County Council said that a review had been conducted at the school to ensure staff followed guidelines and that Mr Byrne would remain off work while a further investigation was carried out.
He said: "The county council is satisfied that, while most regrettable, this was a one-off incident and is sorry for the distress caused to the boy and his family."