But when two teenagers wandered into the playground of Oswestry County (Woodside) Primary School, in Oswestry, Shropshire, after a lunchtime drinking session, the incident could easily have ended in tragedy. One had a knife, both were drunk and abusive and the school was minutes away from releasing nearly 500 children.
"I was working with my deputy head, Elaine Jones, in the office," remembers Rob McDevitt, the headteacher. "We were alerted that there were two teenage girls in the playground. We went out and it became obvious that they had been drinking. One of the girls pulled out a knife and was waving it around, making threats."
Like many other heads, Mr McDevitt had thought about school security after attacks in schools at Dunblane and Wolverhampton. He had been determined to prepare for the unthinkable without turning the school into a fortress. Over several years the building has been made more secure and staff have had training in emergency procedures.
In December last year it paid off. Mr McDevitt, routinely, had taken one of the school's walkie-talkies with him into the playground. After sizing up the situation, he used its open channel to send a message to all the staff in the school. Ms Jones went back inside to see the police were called.
Meanwhile, Mr McDevitt's emergency message had triggered the school's procedures. Teachers came to his assistance in the playground, their classes covered by other staff doubling up. The children were kept inside.
Access to the school is by a secure main entrance door. Some side doors have key-pad access locks and others have no exterior handle. The school was sealed.
"The children knew that something was happening," said Ms Jones. "But we have practised this procedure as a drill lots of times and they thought it was just another practice. They were very calm."
The chair of governors, Alison Moses, came on to the site while all this was going on. "I was unaware that anything was happening, until I was told that I'd walked into the middle of an incident," she said.
When the police arrived, the situation was dealt with fairly quickly. They knew the two teenagers. After some persuasion, both were rounded up and escorted away.
The school's next task was to reassure parents. "We were keen to let people know that at no time were children actually threatened, or in any danger," says Ms Moses.
The local education authority, Shropshire County Council, put out a press release within two hours. Local press coverage was largely accurate, but front-page headlines still shouted about a playground knife threat.
"This was not a violent incident," says Mr McDevitt. "No children were threatened, no parents were threatened."
"The response from parents was wonderful," says Ms Moses. "We had letters, phone calls and an awful lot of people just saying how pleased they were with how everything had gone."
The school's 13 walkie-talkies played a crucial role and, in Mr McDevitt's view, have justified the expenditure. "These walkie-talkies are in key areas of the school," he explained. "With the school manager and senior members of staff. At lunch time they are transferred to playground supervisors.
"The system is checked twice a day to make sure it is working. But they are not used during the day unless there is an emergency."
The school has an agreement with parents that they do not enter the building during the day except by arrangement. When parents pick up their children, they wait in the playground. If they need to speak to a member of staff, they wait for their child, then enter the building, signing in as they do so.
"In the morning parents do come into school with their children," said Mr McDevitt, "But we do ask them to have left by 9am. Generally parents have been very good about this, very supportive of what the school is aiming to achieve."
He argues that the security measures are simply an extension of the school's role in providing a complete education. "It's part of the work we do with children about keeping themselves safe - personal safety means collective safety, collective responsibility."
* In the past two years the Government had allowed certain radio frequencies to be released for short-range radios, allowing manufacturers to develop communica-tions products which are ideal for schools. These fixed-frequency, single-channel radios are effective over 500 metres and have a variety of uses, from playground supervision and car park security watch on open evenings to monitoring during school trips.
* Since March 1 no radio license has been needed for the operation of this kind of walkie-talkie, which costs about pound;170 from retailers.