The union has urged all schools to improve security following the Dunblane killings and the fatal stabbing last December of London head Philip Lawrence.

Measures could also include security cameras, and stopping and searching any intruders.

As well as attempting to prevent a repeat of the Dunblane tragedy, crisis policies should also cover bomb threats, terrorism, natural disasters or hostage-taking, according to the guidance issued at the union’s annual conference in Torquay.

The advice issued to the union’s 32,000 heads and deputies urges schools first to carry out their own security audit on perimeter security, lighting, and controls on access.

Entrance should be limited to one point, monitored by a reception desk, and security could be enhanced by electronic locks, swipe cards or intercom systems.

Unsupervised doors should have self-locking devices and should only open from inside. They could also be monitored by closed-circuit security cameras.

All visitors to schools should be identified with badges. No one should be allowed to wander corridors unchallenged.

Panic alarms and mobile phones should be considered where schools were spread over split sites and staff could encounter intruders while isolated from colleagues.

But David Hart, NAHT general secretary, attacked the Government for failing to make any cash available this year to fund improvements.

The Government’s working group on security recommended earlier this month that extra cash should be provided “at the earliest opportunity”. But ministers ruled this out until next year, saying school security would have to compete with other priorities.

Mr Hart said: “We can find hundreds of millions to compensate beef farmers, but when it comes to urgent funding for school security, the Government doesn’t seem willing to act.”