Buckinghamshire, which tops the exam league tables, could face building costs of at least Pounds 30 million as a result of Gillian Shephard’s ruling that Beaconsfield High School, a girls’ GM school, could lower its entry age from 12 to 11-plus from September 1997.

The Beaconsfield decision followed Mrs Shephard’s ruling in June that some schools in Slough in the neighbouring authority of Berkshire could lower their transfer age to 11 from September 1996. This provoked furious protests as it appeared that Buckinghamshire would be forced to change its system in line with Berkshire, which has poorer exam results.

Both Slough and Buckinghamshire, which were in the same education authority until the last local government reorganisation in 1974, have a middle-school system with transfers at 12. Two selective schools in the south of Buckinghamshire take around half their pupils from outside the county. They stand to lose many of them as they will face competition from schools taking in pupils at 11. So the Education Secretary’s decision has what education chairman Crispian Graves calls “a domino effect across the county”.

The Beaconsfield ruling made matters worse for the authority, especially as it is a girls-only school. Lawyers warned that the council could have problems under equal opportunities legislation because it does not provide the same number of places for 11-year-old boys.

Mr Graves confessed that the consequences of Mrs Shephard’s ruling was “the most complicated thing I’ve had to deal with - it frightens me to death”. Labour’s education spokesman John Huddart called it “bizarre” and “the height of absurdity as it arises from one decision to give a GM school a change of entry”.

An education committee agenda paper says: “The real prospect emerges, unless the authority takes the lead with its schools, governing bodies, the Funding Agency for Schools and GM schools in planning and implementing a ‘planned and orderly’ change, that there will be unplanned and disorderly change to the detriment of the coherence of the Buckinghamshire system and present patterns of parental choice.”

Mr Graves said that preliminary estimates for costs to increase capacity at secondary schools were around Pounds 30 million. Grammar schools are already full. “We are talking to the Department for Education and Employment about our borrowing power and we are going to see Robin Squires (the education junior minister) in a week or so.”

The council is already consulting selective schools in the county and the rest will be included later this term. Milton Keynes, which will become a separate authority in 1997, is also taking part in the consultations. It could face having to build an extra school.

Middle schools are equally concerned about the transfer age because they stand to lose money as their top age-range is funded in the same way as secondaries. There is also the danger of first schools being forced to close if they lose a year’s entry, said a spokesman.

David Whitbread, under-secretary for education at the Association of County Councils, said Buckinghamshire’s situation “demonstrates what we’ve argued: if you fragment the responsibility for the education system between LEAs, the FAS, GM schools and so on, you create chaos.”