In her Washington political debut Ms Feldman said the phenomenon was a major problem in American schools. She blamed it for a lack of reading, writing, and maths skills, particularly among the urban poor.

The American press has reported rising complaints about the number of students arriving in high school and colleges requiring remedial lessons in the basics they should have acquired years before.

No US school district has an explicit policy of “social promotion”, according to a survey by Ms Feldman’s union, the American Federation of Teachers. But nearly half its members say they have been put under pressure to promote students to the next year who were not ready to move on.

Children often progress through the grades simply by putting in time because of pressure from parents, determined their child will not suffer the stigma of staying down a year. And principals are reluctant to halt their progress and make the school look bad.

A small but growing minority of districts have listened to the critics. Chicago, with 425,000 students this summer, required 42,000 to enter summer courses in reading or mathematics or face being failed.

The city of Cincinatti expressly banned social promotion; where before, 95 per cent of pupils automatically moved on, nearly a third were held back in the early elementary grades last year.

It is estimated that between 15 and 20 per cent of American pupils are held back in the same grade in any year. In urban districts, more than 50 per cent of children will be retained at least once.

Ms Feldman praised school districts in Corpus Christi, Texas, and Alberquerque, New Mexico for singling out pupils who are in danger of failing and setting up special support programmes. For struggling students lagging behind, Cincinatti has established “plus classes” with fewer students and intensive teaching methods.

But most school districts tacitly encourage moving pupils up regardless of their grades in the previous year, Ms Feldman said. They do it with rules that forbid holding students back more than once, or holding back those with poor English or learning disabilities.

Feldman used social promotion to make the case for higher standards and the national tests which President Bill Clinton has proposed, which are now running into political trouble (see below).

The problem, she said, is that there are no agreed standards defining what students should know and be able to do at various grade levels. One school in Nevada required only, for example, that “student advancement through the curriculum should be according to the student’s demonstrated ability”.

In Long Beach, California, promotion to the next grade required students to “demonstrate sufficient growth in learning required basic skills”.

In Pennsylvania, the AFT dug up a draft set of required “student learning outcomes” which demanded that “all students understand and appreciate their worth as unique and capable individuals and exhibit self-esteem”. By contrast, national history standards require pupils to explain British imperial policy following the history of Paris in 1763, and the chronology of the outbreak of the American revolution.