More than 13,000 primary school spaces are needed in Essex in the next two years.
The county council has set aside around £300 million to try and meet the demand.
Three years ago at Ravens Academy in Clacton there were 158 children - this September there will be 420.
Karen Hammond, Principal, said: "We have an extra Year 3 class and are being asked to take another Year 2 class.
"That's not just us - that's across Clacton, because otherwise these children won't be schooled.
"Sometimes we have children that haven't been in school for months."
Essex County Council has earmarked £315 million to create 13,000 school places over the next three years.
Cllr John Spence from Essex County Council, said: "It's a big ask and we do need to consider how we keep funding all that investment.
"We are making representations to Government because we should not be left with a huge borrowing burden on the back of this, but yes we can afford to do it."
More than 2,000 places will be available from September thanks to a series of school expansions.