Teachers must be given more backing to restrain unruly pupils through "reasonable force", Britain's biggest teachers' union said today.

The National Union of Teachers called for staff to be trained properly in how to intervene in fights as it launched a charter on acceptable behaviour in schools.

The charter called for pupils who are persistently disruptive in lessons to face expulsion under tougher exclusion powers.

All the major teachers' unions have voiced concern in recent months over increased levels of disruption and violence in schools.

Education Secretary Ruth Kelly has promised a "zero tolerance" approach and Ofsted has warned that lowlevel bad behaviour is getting worse.

The NUT said many teachers worried about being falsely accused of abuse if they used force to restrain difficult pupils.

Steve Sinnott, NUT general secretary, said teachers were reluctant to use their existing powers to intervene physically.

Mr Sinnott said: "Teachers can be reluctant to intervene and it is essential to intervene sometimes because you have got to stop a child from damaging themselves, causing hurt to other youngsters and damaging property.

"We want teachers to be trained to deal with these things with confidence."

The NUT's charter on classroom discipline called for a range of measures to protect teachers and tackle poor discipline. They include:

* Teachers being given a right to have personal panic alarms

* National guidance being provided for training teachers in how to use "reasonable force" to restrain pupils

* The power of teachers to punish badly behaved children being set out explicitly in new laws

* The school curriculum and national testing regime being reviewed because it is too restrictive. Many children behave badly because they have been turned off their studies

The NUT said teachers should be protected by anonymity if they are being investigated following a pupil's allegation of assault or abuse.