No DFEE circular addresses the issue. The relevant 1996 Regulations prescribe a minimum area of playing fields for schools. They are explained in DFEE Circular 1096, available from the DFEE's School Premises Policy Team on 0171 273 5936.
Cash raised from sales of redundant or unusable fields can fund better sports facilities. Whether land sold by a grant-maintained school or by a local education authority is then allowed to undergo a change in use is a planning matter for the local planning authority although, of course, some sales may be dependent on the granting of planning permission.
You must set the record straight. Although there is currently no blanket ban on the sale of school playing fields, it is simply not true that sales are "encouraged" through either circulars or regulations.