The Government has been accused of wasting taxpayers' money by allowing new schools to open in areas where they are not needed.
Brian Lightman, general secretary of the Association of School and College Leaders (ASCL), said such a policy is "deeply and unequivocally immoral".
In his speech to ASCL's annual conference in Birmingham today, Mr Lightman will tell delegates that the union does not object to new schools opening in areas that face a shortage of school places, but "cannot condone the creation of costly surplus places when other services are being cut".
He will say: "We also have serious concerns about the impact of these unnecessary new schools on the quality of education for children in other nearby schools, which are thrown into a downward spiral because of falling pupil numbers and lack of investment.
"Children are not guinea pigs in some educational lab. Schools that have been consigned to the dustbin of our education service in this way cannot be expected to create the conditions which enable them to raise standards.
"At the very least, it's a disgraceful waste of taxpayers' money, but more importantly such policy experimentation is deeply and unequivocally immoral. Actions have consequences and no-one in government should be standing by and watching as some schools fail in order to use it as a lever for change."
Mr Lightman said prior to his speech that there were a "number of areas" where schools are being established but are not needed, including in Suffolk, Brentwood in Essex and Bristol. He also claimed that some new schools are offering services that he believes are unaffordable at current funding levels.
These include offering classes of 20 pupils or less, or teachers teaching for 60% of the time, leaving 40% free for marking and lesson planning. This would be "absolutely wonderful, but we know that would be immensely expensive in terms of school budgets", he said.
New schools are likely to be free schools, although they could also be university technical colleges or studio schools which usually offer a more vocational-led education.
Free schools, a flagship measure of the Government's education policy, are state-funded schools, free from local authority control set up by groups including parents and teachers. The Government says the schools are set up in response to what local people say they want. Some 24 free schools opened last autumn, and more are planned for the future.