In an individual school, this could be explained by a change in catchment area, in the socio-economic composition of the neighbourhood or by the opening of a selective school nearby.
But it is schools in a stable setting that are noticing this trend, and it appears, from a survey carried out among members of the Secondary Heads Association, to be happening across the country.
In my school - a large, popular, successful comprehensive school serving Huntingdon and its rural hinterland - the average verbal reasoning quotient (VRQ) score (one measure of ability) of our 300-pupil-per-year intake has declined from 109 in 1987 to 96.9 in 1995. A score of 100 would be average.
The boys' mean score has gone down even more sharply, from 108 to 92.3 over this nine-year period.
The non-verbal reasoning mean score has fallen from l08 to 100. A similar pattern has been detected in three-quarters of the secondary schools surveyed that were able to give reliable data - some 200 schools.
What is happening? In the primary schools the implementation of the national curriculum and testing have taken up a great deal of teacher energy and time and have arguably made it more difficult to give the necessary attention to basic literacy. Nobody, least of all the Secondary Heads Association, is "blaming" primary teachers. If a finger has to be pointed, it must be at those who insisted on an overloaded curriculum and bureaucratic testing procedure.
But the causes of the decline in literacy may have wider implications. The last decade has seen an increase in single-parent families, in family stress caused by increasing poverty and greater pressure on families in which both parents are working.
These factors mean that young children receive less personal attention in the early, crucial years when they need to be played with, talked to and read to if they are to develop the linguistic skills without which the journey through formal education will be unrewarding and alienating.
Add to this electronic games and videos, and we can see that young children do not have enough time or motivation to work at developing reading.
Children coming into secondary schools who need remedial help must get it. The tendency to move funding down from secondary to primary must be resisted. But we must recognise that more new money must be found to reduce class sizes in the first two years of formal schooling, as well as finding a better way than vouchers to increase nursery education.
We also need a sustained national drive to impress on parents that the early years before a child sets foot inside a school are vital to that child's success.
The writer is head of Hinchingbrooke School and immediate past president of the Secondary Heads Association.Reuse content