This is the academic equivalent of a green ink letter -- a confusion of outrage and opinions without visible means of support. I suspect that there is some truth in there somewhere: my own experiences with the Good Schools Guide suggest that the trend that they are talking about might be a real one. And, like the 110 signatories to the letter, I have my own views on what the causes might be. It is no good, though, letting our educational system being driven by one fashionable opinion after another: we need research.
Research in this area is pretty thin: the article refers to "research by Prof Michael Shayer at King's College, London, which showed that 11-year-olds measured in cognitive tests were "on average between two and three years behind where they were 15 years ago". I will dig up a copy of this paper through the House of Lords library, and see what it says: previous experience suggests that there is a fair chance that the headlines don't match the contents.
We don't have a proper system for commissioning and evaluating educational research. There's so much going on in schools that we could learn from, if only it was documented and tested properly. Lord Adonis (the education minister in the Lords) has hinted that there is something going on in this direction: it can't come too soon.
There's a good BBC comments page on this at http://newsforums.bbc.co.uk/nol/thread.jspa?threadID=3752&&&edition=1&ttl=20060912104507