A member of staff was found dead at Avon Valley School and Performing Arts College in Rugby, Warwickshire, the school has said.
The man in his 50s was discovered as the school was being unlocked on Tuesday morning.
Police have said the death is not being treated as suspicious.
A statement from the school’s Headteacher Alison Davies said: “It is with the greatest sadness that, following police advice, the decision has been made to close the school on Tuesday 29 January, 2019, following the death of a colleague on the school's site.
“Early in the morning, as the school was being unlocked, a member of the school's site team was found dead. The police have been informed and have advised us to close the school for the day.
“We have alerted as many parents and carers as possible that the school will be closed today, Tuesday 29 January, 2019.
“This is a huge shock for our entire school community and our thoughts and prayers go to our colleagues, family and friends at this very sad time.
“The school will be actively offering emotional support to staff and pupils who have been affected by the tragic incident in the days and weeks ahead.”