Barbara White has been awarded pound;12,000 by Malvern College in Worcestershire because she was contracted to work more than 121 hours a week for just over pound;15,000 a year. When her contract was drawn up in 2004 this amounted to pound;3.75 an hour. The minimum wage was then pound;4.50 an hour.

Mrs White worked happily at the school from 1991 to 2004, but found her hours became impossible when she was appointed to the only boarding house without a live-in house mistress or house master.

She was the only person responsible for 30 boarders six nights a week. She had to open the doors at 6.30am and lock up at 11pm, and tend to sixth form girls’ needs throughout the night. The 56-year-old was also responsible for managing a team of catering and domestic staff during the day.

When she asked for an extra night off a week, which the housemistress agreed to cover for her, the school refused. She was eventually forced to leave her post at the co-educational school in December 2006 because it was too exhausting. “I had to sacrifice a job I love for the health and safety of the students. I was tired all the time and spending any off-duty time I had asleep,” she said.

The Association of Teachers and Lecturers, which represented Mrs White, said there was “no excuse” for such levels of exploitation.

Malvern College is a member of the Independent Schools Council. The Good Schools Guide described it as “not an overly sophisticated school”, but praised its house and tutor system. Its elegant Victorian buildings beneath the Malvern Hills are set in 260 acres.

Martin Pilkington, ATL’s head of legal and member services said: “It is just incredible, a school with pupils from some of the wealthiest families in the UK paying its own staff less than the minimum wage.”

He said the case had significant implications for private schools, particularly boarding schools, where staff are often expected to work unpaid extra hours.

Thousands of staff, he said, could be eligible for extra pay, or could have their contracts changed.

He explained that the working time directive applies to schools as much as any other employer.

A recent survey by the ATL revealed that 63 per cent of staff in independent schools worked more than 50 hours a week. It has also sent out guidelines to all independent school heads in the country, explaining how to ensure a work-life balance for staff. Mrs White’s settlement follows a court ruling in 2006, which said that wardens on call in a residential home should have those hours regarded as part of their normal hours, and this should be reflected in their salary.

Ian Barron, the bursar at Malvern College, said following a review of practices, it was now a “contractual obligation” for assistant house staff to finish work between 10pm and 11pm, and they are called out only in emergencies.

He said house staff were competitively paid, received free food and accommodation, 15 weeks holiday a year and a pension. He added: “The claim made by Mrs White was based on the premise that hours spent by her sleeping or watching television in her flat on the college site were working hours. The college, like any other employer, would not accept this.”