Lee Steptoe - who stood against then Tory party chairman Brian Mawhinney in 1997 - quit the party at the National Union of Teachers-sponsored Liberal Democrat education conference last weekend.
Mr Steptoe, said: "Like many teachers and Labour supporters, I was deeply offended by the Government's proposals." He is head of year 11 and of citizenship at Central School in Grantham which has just been approved to become a specialist technology college from September.
He said: "I am happy for my own school and hope the new status will help bridg the divide between schools locally as we are in a selective area. But I have severe misgivings about the wider policy.
"If 46 per cent of schools become specialists, what about the other 54 per cent, which will probably be concentrated in the inner cities? My great fear is that they will become sink schools because they will be regarded as second-rate."
The 31-year-old, who has been a teacher for nine years, said he had gradually become disillusioned with Labour, particularly over the introduction of university fees, but that the Green Paper had been the final straw.
Mr Steptoe joined the party at the age of 14 and was a councillor in Cleethorpes before standing as parliamentary candidate for North West Cambridgeshire in 1997.