Roughly 900 pupils were kept in their classrooms during lunchtime yesterday amid fears of an intruder armed with a knife on the premises of a school.

Staff at Flegg High in Martham took advice from police and immediately ordered the lock-down following reports of a man acting suspiciously.

Lunch-time was delayed by half an hour until police had been called and, following a search and review of CCTV, were able to give the all-clear.

At the end of the school day, principal Dr Simon Fox sent out a note to reassure parents stating: 'By now I am sure you have heard many rumours about an incident that occurred at school.

'In essence, there was a potential safeguarding situation that we took professional advice from the police on.

'The advice they gave us was to hold students in classrooms until the police arrived and we were sure there was no risk.

'You will be pleased to learn that there was no substance to the concern raised; however I'm sure you will agree that in such a situation it is always best to take a cautious approach.

'Again, to reiterate there was no danger at any time to any students.

'I would like to commend the behaviour of students and staff during the situation, it was exemplary.'

A police spokesman said: 'Following a search of the area by officers and a review of CCTV, no male has been traced.

'It is now believed that although the allegation was made with the best intentions, it had no substance to it.'

One mother, whose daughter goes to Flegg, said: 'Outside the school gates it was the talk among parents how they had gone into lock-down with the students not being allowed to leave the classroom.'

She said there were reports the atmosphere was quite tense in some cases and one child who had tried to leave the classroom had been hurriedly stopped by a teacher.