The pupils, aged 11-16, failed a "uniform check", which was carried out at The Dormston School in Sedgley, West Midlands, after they had been warned to drop their hemlines.

"When some of the girls had their blazers on, their skirts were so short you could not tell they had them on," said the headteacher, Barbara O'Connor.

"This is clearly not acceptable at a good school like this."

She said there had been complaints from the local community. A woman who had moved to the area, and had wanted to send her daughter to the school, had changed her mind after seeing some of the pupils around town.

Image problem

"We have really high standards and we were beginning to get a bad name because of these girls," Mrs O'Connor said.

Possible alternative? The new Royal Mail uniform

Twenty other girls were kept together out of class on Monday, but not sent away because their parents could not be contacted to collect them.

Most of the pupils have now returned to school wearing "acceptable" skirts, although eight have not.

Mrs O'Connor said letters had been sent to parents three weeks ago, telling them that girls' skirts could be no shorter than two inches (five centimetres) above the knee.

"Petty"

The shop which supplies the school's uniform has lent a few which pupils can borrow if their parents genuinely cannot immediately afford the price of a new skirt.

"We have had numerous phone calls saying 'well done'. At the parent-teacher association meeting last night they were 100% behind our action," she said.

But one woman whose 12-year-old daughter attends the school, Christine Powell, said the ruling was "petty".

"My daughter's education is being compromised by a silly issue which does not concern her education. I'm not going to let this lie."

Another parent said: "This is quite simply disgusting - what are the school thinking about?"