Children who compare their schools with prisons are nearer the truth than they realise: they are the only places where people are obliged to spend time in close proximity with others whose behaviour, attitudes or other attributes may be harmful.

We are concerned when a custodial sentence is handed down if we think the recipient may learn to be a more effective criminal, may be subject to bullying or abuse or may simply become more inured to a life of crime.

We have been reminded during the past few months of children whose behaviour in school, threatening fellow pupils with a carving knife and assaulting teachers for example, would probably result in custodial sentences in the adult world (The TES, May 12).

There is understandable anger and resentment that schools should have to face such difficulties, but where is the moral repugnance about a society that claims to concern itself with raising standards in schools, but which incarcerates children in institutions where they and their protectors can be subject to such outrageous behaviour?

Social inclusion seems to be the political flavour of the month, and if a symptom of trouble (such as exclusions) can be reduced in size, some people might believe that the causes have been similarly reduced. But pupils who behave so badly that their presence can no longer be tolerated in schools cannot be related to statistical targets. These pupils are real flesh and blood - and whether they behave badly in droves, or in ones and twos, that is the reality of what a school has to deal with.

To tell a headteacher that the school has already used its notional allowance of permanent exclusions is nonsense, but this is what is happening.

When a head has to consider imposing a permanent exclusion, the needs and interestsof the particular pupil are brought to his or her attention by governors, colleagues, the pupil’s parents, the local authority and other interest groups whose help might have been engaged by the parents.

The need to protect the identity and interests of this pupil will mean that the parents of other pupils are unlikely to know anything of the matter. Yet to them a headteacher owes a particular duty of care - legally he is in loco parentis - and any interested parent has a right to know that a son or daughter is not at the mercy of bullies, thieves or persistently disruptive peers.

Heads have very little time to consider this aspect of bad behaviour, and yet it should be paramount when parents are, in effect, obliged to entrust their children to the headteacher and staff of a school and are not privy to information about the company their children may be forced to keep.

Parents should seek assurances that their children will never be forced to sit near, or be left alone with, another child known to have been violent towards other children or adults, or known to be a regular disrupter of classes. Where such assurances cannot be given, heads should welcome parental anxieties as a welcome bulwark against those who might object when the interests of those most vulnerable are put first.

Parents should also remember the arrogance of some of those whom they elect into positions of power, for politicians’ children are often spared from such things. Not so very long ago Shirley Williams, then a Labour secretary of state for education, sent her daughter to an independent school while she was closing grammar schools - which had been provided for other people’s children.

Peter Inson taught in maintained schools for 22 years, the last three as a head in London. He now teaches at an international boarding school in Switzerland