Those who fail to stop after being warned can be arrested. Under the Protection from Harassment Act 1997, belligerent parents can increase the seriousness of the offence if they cause a teacher to fear that violence might be used, even though physical harm does not actually happen.
Unfortunately, for this legal restraint to be effective the teacher has to be harassed at least twice. But a teacher can seek an order restraining the harassment if the parent threatens to repeat it.
It is also an offence to take articles with sharp blades into school without good cause. This covers pupils as well as adults. The police have the power to enter a school to search for a weapon.
Many schools have now introduced signing-in and badge systems. Some are training teaching and support staff in techniques for safe handling of intruders.