A college and prep school have been closed after a sixth-form pupil displayed symptoms of coronavirus.
Brighton College said a day pupil has been tested, but the results were still not known.
The college said a number of other boys in the same day house have also come down with similar symptoms, but they were unable to be tested.
The headmaster, Richard Cairns, told parents: "I have been advised to assume that we have a Covid-19 cluster."
He said the college would be closed for two days of deep cleaning.
"Given the proximity to the end of term and our readiness to deliver a meaningful on-line educational programme, it seems most sensible to keep the college and prep school closed," he said.
Mr Cairns said the prep school shared dining facilities with the college and so would also be closed.
Weekly boarders were also being told they would be leaving the school on Monday with most full boarders having already returned home.
Mr Cairns said parents should "be prepared" for the possibility of the school and college remaining closed following the Easter break.
"I do not know if the college, prep and pre-prep will be open but I think it would be wise to plan for it to be closed just in case," he wrote in a letter to parents.
"This is why we have invested heavily in on-line teacher training so that we can offer a full and meaningful education for each and every child.