Exchanges with foreigners can turn into personal tragedies for pupils and their families, according to Modern Languages, the journal of the Modern Languages Association.

Mr Alan Smalley, the association’s vice-chairman, says that cultural shock aggravated by severe homesickness produces a kind of “exchange syndrome” in a few children.

In extreme cases, they refuse to eat or drink, withdraw totally from social contact and are often found unkempt and unwashed, crouching in a knees-up position sucking their thumbs and demanding to be sent home after only a few days.

Only about 2 per cent of pupils on such exchanges are affected as seriously as this, but those brought to the attention of the escorts might only be the tip of the iceberg.

Worst affected in exchanges to France are usually girls under 15 who are normally quiet and withdrawn. Often they have never been away before without their parents and come from homes where there are rarely any visitors. Frequently their French is poor, which adds to their sense of isolation.

Escorts could often help them, but prevention is better than cure. Better preparation, making sure they at least have a survival kit of basic French, and sending them to homes similar to their own could help them get the full benefit from their visits.

French children in England seem to be less affected. They are often better equipped linguistically and are used to being away from their parents.