Pupils will quit school at 16 instead of going on to the sixth form because of the Government's controversial shake-up of A levels, a teachers' union warns today.

Controversy over the introduction of the new AS-level exams could jeopardise plans – announced earlier this week – to encourage all youngsters to stay on at school or full-time education until 19, says the National Association of Schoolmasters Union of Women Teachers (NASUWT).

In its evidence to the review ordered by Estelle Morris, the Secretary of State for Education and Skills, earlier this month, the union warns that as many as one in ten pupils in some schools is already dropping at least one AS-level subject because of pressure of work.

Some candidates have sat up to nine hours of examinations in one day through taking four AS- levels and studying for the new key skills certificate, also introduced this year.

Pupils have also been put up in hotels or have stayed with teachers overnight in an attempt to avoid contact between them and others who have already sat the exam. The union is recommending a boycott of overnight supervision because of the danger that allegations of abuse could stem from it.

Nigel de Gruchy, general secretary of the NASUWT, said: "Unless this top-heavy and confusing array of examinations is sorted out, pupils may think twice before entering the sixth form."

The union's evidence adds that some sixth-formers are quitting their courses. It says: "In some cases students have become disillusioned with the prospect of more examinations and coursework and are dropping out altogether rather than face such pressure."

The union is urging Ms Morris to ditch the key skills qualification. "Regrettably, key skills have become far too much hassle with too little currency," the union's response says. "Undoubtedly, universities are still very much geared to examination achievement with key skills forming a far lesser priority."