In a letter to parents dated March 13, Hastings-based school Claremont said it would be closing its Nursery, Pre-Prep and Prep school off the A21 in Ebden's Hill, Baldslow, East Sussex, 'with immediate effect' due to a 'positive COVID-19 case associated with the school'.
It said the 'safety and wellbeing of our students, staff and families' was its 'top priority'.
The letter urged parents to keep in contact. It said: "We realise this is an unsettling period and we are here to provide support and advice to you and your child. During the closure period, if your child, you or any of your family members are diagnosed with coronavirus, please inform us as soon as possible."
According to its website, Claremont is a day and boarding school for students aged one to 18 years. Set up in 1925, The Nursery, Pre-Prep and Prep School is set on an 125 acre estate and teaches more than 400 students.
The closure will affect these pupils, and Claremont's senior school pupils - based in Bodiam, East Sussex - who have siblings at the prep school.
The school will initially be closed for seven days, the letter said, while a full deep-clean was carried out.
This followed news yesterday of East Sussex's first coronavirus case.
While the school is closed, pupils will continue their education online.
According to the school's website, the nursery and prep school open morning and forest school experience scheduled today has also been postponed.
The senior school and sixth form open morning was due to go ahead as planned at the Bodiam site.
Last week, the World Health Organization declared coronavirus to be a pandemic. The last figures suggest that more than 125,000 people have been diagnosed in 118 countries, and deaths total more than 4,600.
Four cases have been diagnosed in West Sussex and nine in Brighton and Hove.