A teacher was hauled in front of a General Teaching Council conduct tribunal for misdemeanours including failing to return library books and calling a teenager who swore at him "a waste of space".
Keith Robinson was found not guilty of unacceptable professional conduct by England's GTC at a hearing in Birmingham last week.
His solicitor, Stella Yeomans, said she was surprised a tribunal had been necessary, given the weakness of evidence against Mr Robinson, who had an unblemished record after 33 years in the profession.
The accusations against the 53-year-old related to incidents between September 2004 and December 2005 when he was head of maths at High Ridge Specialist Sports College in Scunthorpe, North Lincolnshire.
An allegation that he had taken library books from the school and refused to return them was dropped on the first day of the hearing.
The committee then considered the remaining accusation, that he had "seriously demeaned or undermined pupils, parents and colleagues by speaking inappopriately to them". But it found the claim unsubstantiated.
In one incident, a pupil swore at Mr Robinson and claimed, incorrectly, that he had lost his coursework. A colleague said he then overheard Mr Robinson tell the pupil he was as a "waste of space". The committee did not believe Mr Robinson's claim that he instead told the student "Get out of my space". But it felt the "waste of space" remark was understandable in the face of such provocation, and, although inappropriate, was not seriously demeaning.
Mr Robinson told the deputy head another pupil had "a great big back and backside". The committee said: "Whilst the remark was wholly inappropriate, it was made in confidence to the deputy head in his office, who then reported the matter to the headteacher".
Alan Meyrick, GTC registrar, said all cases were examined by an investigating committee before being referred to a hearing to ensure there was sufficient evidence.