- Linda Gent, 55, was found at South Benfleet Primary on the evening of Saturday 27 September, 15 hours after she was reported missing.
Initial post-mortem tests failed to establish a cause of death but police said they were not treating her death as suspicious.
Police said they found Mrs Gent's body after being contacted by someone concerned for her welfare.
A police spokesman said: "A search of the school was carried out and shortly after 1800 BST a woman's body was found on the premises."
Mrs Gent had worked at the school since the late 1980s and staff and children were said to be "devastated".
"Everyone connected with South Benfleet Primary School has been devastated by the sudden death of Linda Gent, our school cook," said head Dominic Carver.
"Mrs Gent was a hugely popular and highly-valued member of our catering team."
Officials at education authority Essex County Council said news of Mrs Gent's death was broken to pupils after they arrived at school on 29 September.