Potential sites for a new school for the town will also be considered.
A feasibility study into expanding the two secondary schools in St Neots is to be carried out after the number of future pupils had been underestimated.
Cambridgeshire County Council, which is commissioning the study, had previously been adamant that Longsands and Ernulf schools had sufficient places to cater for nearly 1,000 extra young people expected to come from the new homes being built in St Neots.
Details of the feasibility study proposal were revealed by the county council in the response to a Freedom of Information request about school provision.
Independent St Neots county councillor Derek Giles said: 'I don't like to say 'I told you so' but councillors and members of the public have been questioning this for 10 to 15 years.
Cllr Giles said he welcomed the feasilbility study and added: 'The council has been saying there are sufficient places for years but now they are saying there will be a shortfall and we need to know why it has gone so wrong.'
He said he would like to see a new secondary school built on the Eatons side of the community,
The FoI response, in a heavily censored form, included tables showing planned development: 'They show that 3,815 dwellings are planned in total and that 954 secondary-aged children are expected to move into the town as a result.
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'The current facilities at the schools would be unable to accommodate this rise in pupil numbers.'
Longsands Academy had a capacity of 1,450 pupils, together with a 450-place sixth form, and Ernulf Academy had an overall capacity of 1,160 pupils - but in January 2019 there were just 508 pupils aged 11 to 16 on its roll.
Both are run by the Astrea Academy Trust and had 'requires improvement' Ofsted ratings.
The FoI response said the number of students in the catchment area was likely to increase from 1,974 in 2019/20 to 2,798 by 2028/29 and the number entering Year 7 was forecast to exceed the combined Published Admission Number of 522 in 2023/24 and from 2026/27 onwards.
'Additional capacity is therefore needed to ensure that the council can meet its statutory duty to provide a school place for every child in its area of responsibility and whose parents would like them educated within the state-funded sector without having to rely on places in other schools outside of the town, which could incur high transport costs and a commitment to providing these for the five year period of a child's secondary education,' the report said.
Cambridgeshire County Council spokesman said : 'Currently, there are 600 unused secondary school places across the two secondary schools in St Neots, both run by the same trust (Astrea Academy Trust).
'The significant level of planned housing development within the town, which will increase the number of secondary-aged children within the schools' catchment areas, is fully part of our comprehensive strategy to ensure there are sufficient school places to meet current and forecast need across the county.
'In 2013, councillors agreed the expansion of the existing two secondary schools to meet the future need for additional places. Given the existing current capacity, this is not immediately required.
'A feasibility study is now underway to assess the potential for expansion at the existing schools, which will help to inform the options for delivering the additional places required. This builds on earlier feasibility studies undertaken in the previous decade.'