An educationalist who is to star in a reality TV show about teaching has criticised the GCSE for being too easy.

James Williams, a lecturer in science education at the University of Sussex, is the deputy headteacher and housemaster in the new series of That'll Teach 'Em, starting on Channel 4 tomorrow.

In the show 30 teenagers, predicted to get at least one grade A*-C in science, were selected to spend four weeks doing O-Levels in the fictional 1950s Charles Darwin Grammar School.

He said: "Pupils can achieve top marks simply for noting down key words or two-line answers but if you ask them to explain something they can't because they don't understand it.

"We have pupils gaining A* grades without understanding some of the basic fundamental principles of the subject."

Despite disagreeing with Fifties teaching practices and finding faults with O-Levels, Mr Williams said the experience revealed a fundamental flaw in the GCSE system. He said that while he did not want a return to O-Level papers where pupils were expected to answer in essay form, GCSEs ought to contain opportunities for extended writing where pupils were expected to explain a concept, idea, process or theory.

He said: "Communication, written or oral, is the most important thing we can teach children. It's much more important than details of the periodic table. If they can't communicate, they can't do anything."

Mr Williams, 43, lived and worked at the fictional Charles Darwin Grammar School for a month last summer and taught pupils O-Level science.

He slept in a room next to the boys' dormitory and ate 1950s-style food such as corned beef hash, spam fritters and kippers.

He said: "I was absolutely intrigued by the notion. I went to a grammar school for one year before they were abolished and wondered what it would be like to go back."

He said he modelled his behaviour on teachers he recalled from the grammar school and other bad habits he saw during his 20-year career teaching in secondary schools.

Punishments included getting pupils to polish brass or go for runs at 6am. In one case, a boy who drew an unflattering drawing of a woman teacher was forced to look so closely at the picture he pinned it to the wall with his nose and was left there for 20 minutes.

Mr Williams said: "You'd be hauled before the General Teaching Council for that now. The punishments were totally out of proportion. I'm not that nasty in real life."

A spin-off series, That'll Test 'Em, pitches pupils from the show against their parents in a quiz due to start on April 25.