A teacher told a court how she felt "intimidated, frightened and upset" when confronted by a parent who was angry about his son's torn trousers.
Heather Bennett, head of PE at Ysgol Glan Clwyd - a Welsh- language secondary school in St Asaph, Denbighshire - said she was threatened by Desmond Williams, who told her: "If you don't bring me money for these tomorrow, I'll come looking for you personally."
She told magistrates at Prestatyn that she had never come across such appalling behaviour in 28 years of teaching.
Mr Williams, 43, was found guilty of using threatening words or behaviour following the clash with Mrs Bennett in the school's reception area on February 12.
Mr Williams said he called at the school to find out why his son had been made to take part in PE the day before when he had a sick note for a chest complaint. During the lesson, the boy's trousers had split.
Mr Williams denied raising his voice and said he had not threatened Mrs Bennett.
But chairman Richard Welch said the fact that he had admitted saying he would go looking for her was enough to cause alarm.
Mr Williams was fined Pounds 150 and ordered to pay Pounds 165 costs.