Woodstock in Ontario, Canada, is smaller than Bury St Edmunds in Suffolk, but last week it was shocked to find it had big-city problems after a shooting on its college campus.

Last summer, a gunman at Phoenix Community College in the United States wounded three people. In September, in Finland, trainee chef Matti Juhani Saari, dressed in black and wearing a ski mask, shot and killed 10 people at his vocational college before turning his gun on himself.

The spectre of the “hostile intruder”, armed and firing indiscriminately, has not yet visited Britain’s colleges. The 1996 massacre at Dunblane Primary in Scotland in which 16 pupils and their teacher were shot dead stands as a rare example of a gunman invading a UK educational institution.

But as part of efforts to tackle violent crime and ensure that colleges remain safe havens in sometimes troubled communities, principals are learning how they might prepare for the worst.

Peter Smith, a former detective superintendent with Lancashire Constabulary who headed Britain’s search training centre, where the police and military are taught how to conduct searches of the public, is running a series of courses with the Association of Colleges to advise staff how to deal with all kinds of armed threats, from kitchen knives to firearms.

“What do you do when the principal or member of staff in charge is told there is someone walking around with a gun or a samurai sword?” he asks. “It has happened abroad, but not here to that extent. But it is probably just a matter of time.”

Hitting the fire alarm can be the worst thing to do, sending panicking students into the corridors where they could be in much more danger. Instead, Mr Smith advocates a “lockdown”, as practised in the US in the wake of the Columbine High School massacre where, in 1999, two students shot dead 12 others and a teacher.

Classrooms should be designed so that they can be locked from the inside, with no glass that can be broken or fired through. And there should be a distinct alarm - different from the one that signals a fire or the end of lessons - to warn teachers to keep students in class until the danger has passed.

But knives present the more imminent risk. Home Office research suggests that 500,000 teenagers are carrying blades. They need no training to use them, they are cheap, and they can be found in every home. Mr Smith says small paring knives taken from kitchen drawers are among the weapons most commonly carried by young people.

Despite these figures, many students say their colleges are the place they feel safest, and teenagers are far more likely to become victims of violence off campus.

“What happens out there in society . there is a cross-section that is reflected in schools and colleges,” says Mr Smith. “Students and young people come from outside and bring their problems with them - they don’t necessarily stop at the front gate.

“But if you look at gang cultures, they sometimes treat schools and colleges as neutral territory. You can have colleges right in the middle of different areas claimed by gangs, but they all come to the college and treat it as a neutral area.”

But attacks do happen. Last February, a student was treated for stab wounds to his upper body after a knife attack at Croydon College. In response, the college allowed police to search students using metal detectors. Walk-through metal detectors - or “knife arches” - have also been used as a temporary measure by some colleges to deter students from carrying weapons, and sometimes to provide reassurance to the school community.

Several principals say that using such devices revealed that knife- carrying was far more rare than students or parents might fear.

But British colleges are unlikely to follow their US counterparts in installing metal detectors permanently. After all, airport-style security means airport-style queues, and few would find it practical for college life to grind to a halt as students empty their pockets and file in through narrow channels.

The training aims to help colleges use random screening as an effective deterrent. Since 2007, they have had powers to screen students with metal detectors and to search them without consent if necessary. But many lecturers have misgivings about this, feeling it is a long way from the educational ideal.

“A principal can’t order them (teachers) to do it,” says Mr Smith, “it is voluntary. We have to persuade people it’s about their safety and other people’s.”.

Finding weapons on an individual is relatively simple. Although teenagers might conceal them, they have to keep them accessible, so pockets, bags and tucked into the waistband of jeans are the most common hiding places.

Less simple is working out who might be carrying a weapon. “The big mistake lecturers make is stereotyping,” says Mr Smith. “It’s easy to turn round and say, `There’s a young male. He may be a member of a gang from the way he’s dressed. He looks like he’s carrying a weapon.’ But it’s just as likely to be a quiet student who may be being bullied or in fear of other people with weapons.”

Colleges are caught between two fears. If they take a low-profile approach, they avoid stirring up unnecessary fears but risk criticism should the worst happen. If they do take measures against knives and guns, the public - which already tends to overestimate crime rates - assumes that weapons are rife.

In fact, knife crime remained at a stable rate last year - at around 7 per cent of all crime, despite a series of high-profile cases.

Mr Smith says that colleges face a battle as much against public perception as the relatively rare instances of knife and gun crime.

“We have to turn that around,” he says. “Being a responsible college means they have to be prepared.”

For more information, visit www.statustraining.co.uk.