Education bosses have been forced to scrap plans to extend Peacehaven Secondary School. It means many local children won't be able to go there in the future.
It had been hoped the school, which can accommodate 750 pupils, could be extended to take up to 900.
But East Sussex county councillors have been told the project would cost at least £2 million more than the £5 million they had budgeted for.
Denise Stokoe, director of education, told a meeting of the ruling Cabinet: "We are very conscious a decision to not go ahead will have very serious effects for the school and the community.
"It will be very disappointing and will mean some children in Peacehaven in future years will not be able to go to the school.
"But it's my professional view this scheme to expand the school doesn't provide value for money."
The school had been built through a Private Finance Initiative (PFI) with a private sector partner called Peacehaven Schools Ltd, which is understood to have priced the extension at £7 million.
Because of the PFI deal, education officials cannot seek a cheaper quote.
Government leaders had originally agreed to provide 750 places and there was an option of extending, provided it passed certain value-for-money tests.
After 18 months of negotiations between the council and Peacehaven Schools Ltd, the scheme has been deemed too expensive and the council wrote to parents before the summer break explaining what had happened.
Councillor Rupert Simmons, lead Cabinet member for education, said: "Without PFI there could have been no new secondary school and no new primary school buildings in Peacehaven.
"But PFI in effect leaves us unable to go to the market to get the best price for the extension. I am dismayed and angry we are left in this position."
Councillors looked at other options for the school to accommodate extra pupils as intake increased over the next few years, including the use of temporary classrooms costing £400,000.
Councillor Kathryn Field, opposition councillor for education and libraries, said: "I am very disappointed to hear this recommendation because we struggled for years and years to get this school.
"The knock-on effect for the community is going to be huge."
They agreed negotiations to extend the school should cease and an application to reduce the admissions numbers made.
On top of this, providing help with the cost of transport to children who did not get a place in the school from 2004 should also be looked at.
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