For generations, adults have argued with children about what they should wear, and are neither consistent nor rational. In the 1960s and 1970s, boys wanted to wear their hair long and schools told them to cut it. Recently, shaven heads came into fashion, and boys were ordered to grow their hair.
Bare thighs are usually deemed indecent for girls, but were once obligatory for boys. Girls want to wear trousers when skirts are required, skirts when trousers are required.
The arguments have nothing to do with aesthetics or decency. They are about adults imposing authority and children defying it.
It may be cynical and insensitive of me to say so, but I suspect the case of 14-year-old Sarika Watkins-Singh - the Sikh girl who wished, contrary to a "no jewellery" rule at Aberdare Girls' School in Wales, to wear a kara (steel bangle) on her arm - began with a child's wish to assert herself against authority. Sarika's parents do not seem strict Sikhs; the decision to become observant was hers, taken after a visit to India three years ago.
The kara is one of five Ks, or articles of faith, demanded by a guru in 1699. Only fully-baptised Sikhs are required to keep all five. Most keep only one or two, usually the kara and kesh (uncut hair). Still, in parts of Punjab, where most Indian Sikhs live, about 80 per cent of young men cut their hair. No doubt they, too, are defying authority.
Sarika's case illustrates how three developments have made dress rules more problematic for schools. The first is the decline of deference among adults, many of whom are now as keen as their children to challenge authority. Schools could once count on support from nearly all parents. That is no longer so.
Second, religious commitment, often expressed through dress, ornament or grooming, has become more important as an assertion of identity, especially among minorities. And, third, the law is brought into educational disputes far more often. The case of George Archer-Shee, expelled from naval college for stealing a postal order in 1908 - the inspiration for Terence Rattigan's play The Winslow Boy - became a cause celebre because it was then so unusual for a parent to take an expulsion to court (and win), as Archer-Shee's father did.
Some commentators argue Mr Justice Silber was wrong to find in Sarika's favour. They say anti-discrimination laws should ensure all citizens are treated equally and remove irrelevant criteria in, for example, decisions about employing women, gays or blacks. In Sarika's case, they argue, she will be treated unequally because she alone - the only Sikh in a school of 600 - will be excused the "no jewellery" rule. It is wrong to discriminate on grounds of religion, they say, but also wrong to make religion a reason for excusing someone from rules that apply to all others.
As an atheist, I sympathise with this argument. But I fear things are not so simple. Migrants and their offspring suffer hatred, prejudice and discrimination. They often feel upset and puzzled that they are not wholly accepted as British and even more upset, after a visit to their homeland, to find they do not belong there either. To them, religion can be a source of solace and identity, and they assert it more strongly for that reason. They are encouraged in this by faith "professionals" - priests, elders, imams - who see a chance to increase their power and standing.
School dress codes once channelled adult-child conflict into a relatively harmless area. Now they involve dangerous territory. Much as it goes against the grain, schools must apply them flexibly.
Peter Wilby, is former editor of `The New Statesman' and `The Independent on Sunday'.