A technology teacher who planted a fake bomb as a prank on a colleague, leading to their school being evacuated, has been reprimanded for unprofessional conduct.

Geraint Lloyd Jones admitted he had acted “mindlessly and stupidly” when he placed the device, made from a 12-inch package with black insulating tape, a bleeping digital stopwatch and protruding wires, in a fellow teacher’s bag.

A professional conduct committee of the General Teaching Council of Wales (GTCW) sitting in Ewloe, Flintshire, heard how the joke went “disastrously wrong” when the school was evacuated and the police arrived.

The incident happened in February 2008 at Ysgol y Creuddyn, a Welsh-medium comprehensive in Llandudno, north Wales.

Mr Jones, 29, told the hearing that he had planned to play a joke on fellow teacher and best friend Richard Williams. He had the idea after a bomb hoax had been received by the school by phone that morning, Mr Jones said.

He put the package in Mr Williams’ bag hoping he would open it at lunchtime in the office they shared.

In fact Mr Williams had gone to another part of the school for a geography lesson. He opened his bag, heard a beep and saw the stopwatch, and the alarm was raised.

Mr Williams said that had he opened the bag in the office in the presence of his friend it would have gone no further. “It was a joke between two friends,” he said. Mr Jones said he wished to apologise to everyone concerned.

Headmaster Dr Meirion Davies described it as an “immature and stupid joke” but said Mr Jones was a “conscientious and popular” teacher who had achieved fine results for his pupils.

He had not been suspended because of his work in preparing pupils for important exams, but was given a written warning. He has since been promoted to head of department.

Arfon Williams, the school’s chairman of governors, praised Mr Jones’s teaching and said he hoped he would remain at the school “for quite a few years yet”.

Mr Jones was given a formal caution by police after the Crown Prosecution Service decided not to pursue the matter through the courts.

Dr Davies recalled going to the classroom and deciding to sound the alarm and evacuate “in case it was a real device.”

The committee agreed there had been unprofessional conduct.

Chairman Richard Parry Jones said that it had been “very irresponsible,” particularly as the headmaster had called a special staff meeting that day urging teachers to be watchful following the earlier hoax phone call.

He said Mr Jones had failed to consider the potential of his actions which could have led to fright or panic.

“Teachers including Mr Jones should recognise the risks in playing a practical joke that could cause panic in the school or be against the safety of the public,” he said.

“He should have known there was an element in which he could have caused distress. The public expect teachers to be responsible and give leadership and guidance.”

Announcing the reprimand, which will be in force for two years, he said: “The committee have taken into consideration the previous good record and that there won’t be any repeat of this kind of behaviour.”