A teacher was ordered to retrain for spanking a teenage pupil on the bottom and calling her a "naughty girl."
Science master Trevor Towers, 49, crept up behind the 14-year-old girl as she bent over a school printer.
A hearing at which Towers faced being struck off as a teacher was told that he slapped the girl in full view of her classmates.
The sobbing girl said: "I was in my science class being taught by Mr Towers in the computer room when I went to the printer.
"I had to lean over because it had jammed when I felt a firm slap and he said, 'Naughty girl.'"
"He was standing directly behind me. I didn't know where to turn.
"I felt incredibly embarrassed he had done this to me in front of my classmates. I could feel myself turning bright red.
"I couldn't believe what had happened. I couldn't understand what I had done to make him pick on me."
The girl, identified only as Pupil A for legal reasons, walked out of Trevethin Community School in Pontypool, South Wales, and told her mother what had happened.
A disciplinary hearing of the General Teaching Council in Cardiff heard that the girl's mother went to the school to confront Towers, who apologised after describing the classroom incident as "harmless banter."
The mother later made a formal complaint to Gwent Police and Towers was charged with indecent assault.
He was acquitted by a jury at Cardiff Crown Court in September 2005 and agreed to be bound over to keep the peace in the sum of £250.
Towers was sacked by the school in January 2006 after an internal disciplinary hearing found him guilty of gross misconduct.
Damien Phillips, of the GTC, told the hearing: "Mr Towers admitted he slapped a 14-year-old girl on her bottom in full view of her classmates.
"As a result she was exposed to considerable distress and felt compelled to report the matter to the police.
"Throughout the various proceedings he has demonstrated a consistent lack of insight into the seriousness of his actions, which is of great concern considering he is a teacher of 22 years' experience."
Towers, who did not attend the hearing, was found guilty of unacceptable professional conduct.
But the panel decided to allow him to continue teaching if he completes a retraining course to update his knowledge of safeguarding children.
Chairman Jacquie Turnbull said: "We were guided by the fact that the crown court dealt with this on the basis that it did not amount to criminal conduct. It was a single event in 22 years of teaching."
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