Ten million French children returned to the classroom yesterday to find their lessons crammed into a four-day week — a revolution that delighted families but drew criticism from experts.

In a scheme decreed by President Sarkozy, all primary and junior secondary children are being spared the unpopular tradition of Saturday morning classes. Since most schools are closed on Wednesdays, the majority will enjoy three days off school every week.

No other Europeans, except for a small minority in Germany and Luxembourg, follow a four-day week. French Lycée (senior secondary) pupils continue with Saturday classes. Children will still spend as much time in the classroom as the European average, but educators say that their learning faces disruption by being squeezed into two blocks of two days.

“They took no account of scientific research,” said François Testu, a lecturer at Tours university and the author of Life Rhythms and School Rhythms. “Children need a rhythm and the four-day week creates breaks. It is doubtless a decision that pleases parents but they do not realise the damaging consequences,” he said.

The new system was cheered by parents and teachers when it was announced a year ago in fulfilment of an electoral promise by Mr Sarkozy. Saturday school had long been cursed by families who have to rise early to escort children and forgo weekend trips.

It meant that divorced parents with weekend visits spent less time with their offspring. Teachers also disliked the two-hour Saturday session, which ate into weekends: quite a few played truant themselves.

Xavier Darcos, the Education Minister, originally wanted schools to make up time with classes on Wednesday mornings but most local councils strongly resisted the idea, which would have required them to spend more on transport and catering.

Wednesdays are traditionally for sports and recreation. The fractured routine, unique in Europe, dates back to the days when Thursdays were devoted to Catholic instruction and children attended school all day on Saturday.

The teachers' unions, which are at war with the Government over staff cuts and other reforms, are predicting chaos because they must now also give new classes to underperforming children.

Mr Darcos, whose ministry employs more than a million people, ordered the extra classes to remedy the failure of about 20 per cent of primary school leavers to meet minimum literacy standards.

The unions, which are heavily left wing, and the Socialist opposition say that the end of Saturday classes will hurt poorer children because they will be left to their own devices rather than engaging in constructive recreation. “This free time will enable the children of the privileged to perfect their education, but what about the others?” asked Jack Lang, a veteran Socialist Education Minister. “One of the effects will be to widen the social and cultural gulf between children.”

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