No basis for grammar slur
I am aware of two sets of pronouncements by Professor Jesson on the subject of grammar school added value. Neither of them yet merits the term "study"; Professor Jesson has not published any serious academic work to support these claims, and indeed refuses to share the basis of his judgments with fellow academics, like Dr John Marks, who might expose their deficiencies.
His most recent pronouncement - the main subject of your story - is based on the much derided "progress measure" proposed by the Department for Education and Employment for last year's performance tables.
This measure was dropped in the face of the overwhelming oposition of the teaching profession. It was too crude to be used to draw any proper conclusions.
Schools were put into "baskets" of similar types, measured by their school-level key stage 3 performance.
They were then categorised according to their GCSE performance. Only the top 25 per cent were considered to add significant value.
Since well over half of grammar schools were put into one "basket" it was impossible for more than a few to receive this distinction.
Most people accept that for value-added judgements to be respectable they need to be made on a pupil-by-pupil basis. I am amazed that Professor Jesson should use such reviled data so crudely and then make such categorical judgments about it.
Caistor grammar school