If all efforts to control a situation through good behaviour management have failed, in extreme circumstances you can use physical means to prevent injury.
Article 4 of the Education Act 1998 enables a member of staff to use reasonable force to prevent a pupil committing an offence, to prevent him causing injury to himself, to anyone else or to property.
A member of staff can mean a teacher, classroom assistant, caretaker, supervisor or any other staff member given instructions by the principal to look after pupils.
http://www.deni.gov.uk/teachers/h_safety/reasonable_force.pdf
If you are aware of a pupil who is at risk of needing restrained then the principal and Board of Governors should plan for such an event. Parents should be involved and should know what action the school might need to take. All staff should know what action to take.
Reasonable force is the minimum necessary to prevent the injury. Due regard must be paid to the dignity of the pupil. It is important to take into consideration the age, size, understanding etc. of the pupil.
Reasonable force should be a last resort when nothing else will defuse the situation. E.g.
Pupils attacking each other or a teacher.
A pupil in danger of causing injury through using dangerous materials.
The use of reasonable force in these cases might be the interjection of a teacher between the opposing pupils or teacher and pupil, holding, leading a pupil by the arm, blocking the path of the pupil or by shepherding away with a hand in the small of the back.
Restraint should only be used if the pupil is incapable of self-restraint and they should be told throughout that as soon as they calm down they will be let go.
If restraint has been used it is vital that a detailed, written record is made and a copy given and discussed with parents.