Hundred of Hoo school in Rochester, Kent, is a comprehensive in grammar territory and is one of six secondaries that took part in the “Working inside the black box” study.

The transformation started with science and maths lessons, where assessment for learning was first put into practice.

Science teacher David Tuffin said marks out of 10 were withdrawn and replaced by detailed comments.

Far from increasing his workload, he found using careful and detailed feedback on work enabled him to recall immediately each student’s progress. Not one pupil commented that scores no longer appeared on their work.

More sophisticated questioning worked wonders.

“At times I feel more like a chairperson than a teacher. Pupils interject at any point. I comment on what they say and a pupil will add another point,” he said. “It has been the most motivating, stimulating experience of my career.”

The blanket school policy of giving marks every two weeks has become less rigid. Faculties can chose what works best for them and although summative assessments are recorded regularly, pupils are not necessarily issued with marks.

Far from allowing staff to become sloppy, assessment for learning forces teachers to focus more closely on what they are doing, Mr Robinson said. “If teachers are not always using a red pen, they have to use something else instead, which has meant pupils are more involved in their own learning. Self and peer assessment has been particularly successful with lower ability pupils.”

Two years ago only 30 per cent of pupils gained five or more A* to C grade GCSEs but this rose to 42 per cent last year, following the adoption of assessment for learning.

Mr Robinson expects pupils to reach 47 to 48 per cent this year, just below the national average.