The reasons, highlighted by research done on behalf of the Kent Schools Farm Forum by Wye College, University of London, are mainly to do with the introduction of the national curriculum and local management of schools (LMS).
The Wye College research report suggests that by 1912 there were 2,500 "school educational gardens", a number which increased hugely after 1940 as the result of the government's "Dig for Victory" campaign. Their importance continued during the Sixties, when "Rural Science" became a popular secondary school subject.
Now, rural science has gone, and there is little in the national curriculum which directly justifies having a farm in the school grounds. Added to this, under LMS, local authorities are not so willing to employ qualified livestock keepers. As the result of these pressures, there are probably now only 116 school farms left across the country.
Yet, as the Wye College report points out: "School farms provide a superb educational resource for use across the curriculum . . . they enable pupils to work collaboratively, with physical space, a real-life context, and sense of purpose."
The report, which is only guardedly optimistic, warns of the kind of difficulties which led to the decline in the first place - national curriculum means that farms "risk being seen as 'sweet' projects for the less able pupils".
The farms, too, have to be run properly, all the year round, by people who know what they are doing. Anything else is counter-productive. "If pupils perceive the farm to be poorly maintained or cruel, the presence of the farm simply causes anxiety amongst pupils and concern amongst parents."
Done properly, though, a school farm can attain educational excellence. The Office for Standards in Education report on Astor of Hever school in Kent, which has one of the county's 14 farms, says: "The farm is one of the most effective educational resources seen in a school since starting the OFSTED inspections . . . everything observed at the school farm was very good or better."
The conclusion implicit in the Wye report is very clear - if a school is going to run a farm then it had better do the job properly, with clear educational objectives, using the very best techniques.
The Value of School Farms is available from Harriet Festing at Wye College, University of London, Wye, Ashford, Kent TN25 5AH or by fax 01233 813498. Pounds 5 including postage.
u Does margarine come from cows? Is spinach grown in this country? The farming community is increasingly concerned that many children, according to national surveys, appear not to know basic things about their food.
As part of their attempt to put this right, this summer the National Farmers' Union is running a series of "Friendly Farm Fundays" at more than 120 farms across England and Wales.
According to Tony Pexton, deputy president of the NFU, "visitors will be able to watch cows being milked, sheep sheared, go on tractor rides and talk to farmers about life on the farm."
For details of participating farms and opening times, send a 25p sae to NFU Public Affairs Department, Agriculture House, 164 Shaftesbury Avenue, London WC2H 8HL.