More than 3,000 of the 19,000 school places required have been readied in time for the new term.
Essex County Council is investing £315million in creating new school places before the end of the year.
So far £75 million has been spent on completing 2,325 primary places and 1,110 secondary places in time for the start of the 2018/19 academic year, with 510 further places available following the completion of two more projects in Chigwell and Hockley later in 2018.
The rest of the cash will be used to make around 7,500 more primary and secondary places available by this time next year, along with 400 new special school places.
The expansion goes towards the 19,000 extra places required by the 2021/22 if the county's various house building Local Plans go ahead.
Cllr Ray Gooding, cabinet member for education, said: “We have a very strong track record in investing in school places across the county which is the reason why, year-on-year, we are able to offer so many parents their top preference of school for their child.
“Each project completed this year has been carefully selected and with the demand for places only set to continue moving forward, we are working hard to ensure we are prepared to meet it, particularly in rapidly growing areas such as Colchester.
“I am delighted we are able to welcome so many pupils back to new and improved school facilities this year and would like to thank everyone involved in planning and delivering these projects - I look forward to continuing our work in the future.”
The improvements in place this Autumn include an 150 space expansion of Roding Valley High School in Loughton.
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