A large number of pupils in the United States are preparing to go to school during their summer vacation.

Summer schools are growing increasingly popular in the US, as school districts come under growing pressure to end the "social promotion" of students who are not academically ready to move up to a higher grade with their peers.

In New York City, 250,000 students are expected to attend summer schools this year. This is a quarter of the city's total number of pupils - and seven times the number who attended last year.

The Columbia school district estimates that 22,000 of its 71,000 students will return to classes when the academic year ends next week.

In Detroit, between 30,000 and 35,000 pupils, out of a total of 167,000, are expected to attend compulsory summer schools. Last year, the scheme was voluntary.

Financial strain

Nearly a third of the 15,000 school districts in the US require children who fail certain standardised tests, receive low marks from teachers, or skip school regularly, to attend summer school classes.

Twice as many offer special classes, summer camps and fun activities for both younger children and high school students.

But as enrolment in summer schools rises, some districts are struggling with the cost of providing them.

This year, Detroit's compulsory five-week summer school programme will cost $12m for staff and resources - about four times the cost of last year's classes.

Spokesman Stan Childress said: "It has been a tremendous strain on the budget, but we have to take the first step to end social promotion."

'This is not baby-sitting'

The Columbia school district's summer school budget has been cut from $14m last year to $9m this year, as part of cuts to school spending across the board.

As a result, its summer schools have been shortened by a week, and some training for staff has been reduced.

Mary H Gill, deputy chief academic officer said summer school programmes should be considered essential.

"This is not baby-sitting by any means," she said. "Everyone can't afford to send their children to the best of camps or other things that are going to enrich their minds.

"We are trying to make sure the growth that's made during the year is sustained. It's good use of our tax dollars."