Occasionally, I help out at my daughter's primary school, and I have to admire the teachers' patience. When 30 little souls are pressing you to sharpen pencils, find handkerchiefs and wash inky fingers, it must be difficult to keep your cool.

However, once in a while we hear stories of teachers who abandon positive encouragement, are driven beyond sarcasm and threats, and finally enter the zone of cruelty.

We may think that petty power games have been banished along with the cane, and that our children will never be subject to ritual humiliation or unnecessary controls. But we would be wrong.

Recently, a 38-year-old teacher from Harlow, Essex, was discovered to have sealed the mouths of her nine- and 10-year-olds with masking tape. Apparently, she started with one noisy child, then threatened to gag the rest of the giggling class, and they took her up on it. Towards the end of the lesson, when the children tried to remove the tape, the joke did not feel quite so funny.

I asked my teaching friends whether such practices often went on in the modern classroom. I was unprepared for the candid nature of their replies. Besides the usual punishments and controls (writing lines, standing the child in a corner, fingers on lips), they gave vivid descriptions of the unsanctioned tricks of their trade.

'Most kids know their rights very well,' said a teacher with experience in and out of the state system. 'They say: 'You can't touch me. If you do I'll get my mum on to you.' In public school, they tell you: 'I'll call my lawyer'.'

So hitting is out. But masking tape is definitely in. One teacher in a Catholic school taped together the hands of a 'fiddling' child to help him see the error of his ways. The boy turned up for school dinners with his palms still taped together.

Another recalled a boy who used to chew books. His teacher said: 'If you chew your books again, you can come out to the front and eat this piece of kitchen paper.' The boy, unable to break his habit, duly digested the whole square.

'The trouble,' commented a deputy head from Cambridgeshire, 'is that the children take you at your word. The teacher makes an idle threat, and the children take her up on it. Suddenly her back is against the wall.'

Kitchen paper reappeared in a story from a Church of England infants school in Lancashire. A four-year-old boy was heard to utter the word 'bum', and for this his teacher washed out his mouth with soap and kitchen roll. When the rest of the class complained to their parents, the incident was denied. The teacher repeated the treatment later that term on another child.

Sometimes sanctions go wrong. One of the simplest is to send an errant child out of the room. A few teachers I spoke to had been dismayed to find that the child had then been forgotten. 'You send them out of the class, and then half an hour after school's finished, when everyone's gone home, you find this poor child still standing patiently in the corridor.'

In one class, I witnessed a tiny five-year-old being told to stand up because she was not listening. But she was not much taller standing than the rest who were sitting on the mat. When she began to loll after 25 minutes, the teacher blushed; 'Oh yes, Nancy, you can sit down now.'

By the time children graduate to senior schools, measures have become more drastic. But the motives - of control and punishment - are broadly the same. One maths teacher subjected disobedient boys to 20 minutes' torture on a ledge. He would place the offender on a shelf, 3ft high and less than 3in wide, which ran around the classroom. The boy would be forced to balance on his heels until he was brought down.

One English teacher believes cruelty is more institutionalised in traditional public schools than in most state schools.

'I think it comes from wearing gowns and calling children by their surnames, which sets up a distance between pupils and staff. Children in state schools would be extremely annoyed if you started calling them 'Jones' and 'Smith'.'

She recalls a 'pretty obnoxious' public school pupil who was campaigning to be head boy, and who kept turning up at the staff room door. One day he was answered by the history master, who coolly dismissed him with 'F--- off.' This was generally agreed by his colleagues to be a witty way to deal with the child.

Meanwhile, a young female teacher in a tough inner-city comprehensive was dealing with a particularly difficult pupil. 'The deputy head said: 'Send him to me, I will rough him around in my office, and if anyone questions it, I'll just deny it. No one will ever know.'

'I didn't send the boy, for fear of what would happen to him. But that kind of thing does go on in rough schools. They seem to know how to hurt someone without it showing.'

When asked about positive forms of discipline, all the teachers I spoke to were well aware of the best practices: using eye contact, handing over responsibility to the child, avoiding verbal abuse and threats. Table points, gold stars and 'circle time' discussion sessions were all described as ways to engage a child's co-operation.

But teachers admitted that some of their colleagues controlled their classes with a 'barrage of abuse', threats and fear. So long as they were not caught brandishing the masking tape, these teachers expected to get away with it. Teachers who do not hit may still choose to hurt and ridicule.

Our children may not always be little angels, but they have a right not to be ruled by humiliation and fear.