A power cut has closed White Court Primary School in Great Notley, Braintree.
Headteacher Sue Spittlehouse said she has been told there is a phased failure, which is causing a short circuit.
The exact cause of the power cut is still being investigated.
Mrs Spittlehouse said: "No one can give a definite answer at the moment.
"We were completely blacked out this morning.
"I feel the problem could be part of the building project we had.
"Parents were told very early this morning - I had the message on national radio.
"I hope we will get the all clear today.
"If it's only a small area there are some things we can do."
Mrs Spittlehouse said she hopes the school, which has 559 pupils, will reopen tomorrow.
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