A teacher went 'ballistic' and hit a pupil when the teenager took the mickey out of the beard he had been growing for charity.
The 14-year-old pupil held up a crudely drawn cartoon of the facial hair and said 'This is you sir' to Elgan Wyn Jones. Sir's response was to strike the pupil on the back making a 'slapping sound'.
The 59-year-old, who was working as a supply teacher at Preseli Comprehensive School in Crymych, Pembrokeshire, called the pupil a 'waste of space' and 'stupid' during the incident, a tribunal heard.
The art and design teacher had grown the moustache and beard as part of the Movember campaign to raise awareness of testicular cancer.
The hearing at the General Teaching Council for Wales in Cardiff was told Mr Jones sent the boy out of the class to control his behaviour but allowed him back provided he 'calmed down'.
The incident occurred later in the lesson when the boy held up the drawing.
Presenter Gwenno Hughes-Marshalls said: "At that point he ordered him to leave the classroom and hit him on the back.
"It was hard enough to make a slapping sound heard by his classmates."
In a statement, the boy, who suffers from attention deficit disorder, said: "He said I was a bloody waste of space and that I had no brain and I was stupid."
Jones, who formerly headed the school's art department but has had to take up work as a builder following the incident, denied hitting the child.
He told the hearing: "The boy drew attention to himself by laughing at my moustache and beard which I was growing for the campaigns to raise awareness for cancer.
"He held up the drawing of me and I pointed at the door and just touched him with my arm - there was contact but no force."
The hearing was told that police investigated the incident but decided not to refer the case to the Crown Prosecution Service.
Mr Jones was found guilty of unacceptable professional conduct and was reprimanded by the council's professional conduct committee.
Martin Hopwood, representing the teacher, said: "He has an hitherto unblemished character, a reputable teacher with a previously exemplary record.
"He has been unable to earn a living as a supply teacher since the end of 2011 and has had to find alternative work as a builder."
Conduct Committee chairwoman Rhiannon Steeds said: "All teachers can face challenging behaviour and should not react in a way that results in physical contact with a pupil.
"The reprimand is served to maintain public confidence in teaching as a profession."
Mr Jones can remain on the Register of Qualified Teachers and can continue to teach.