Eton College reopened this week after being shut because of a swine flu outbreak that affected 31 people, including 18 pupils.
Andrew Wynn, the college bursar, said that pupils were still being monitored for symptoms as they returned on Sunday evening.
"It (the outbreak) started at the beginning of half term, so the pupils had already planned to go home," he said.
"That was a lucky feature. We heard that a boy had tested positive about an hour after all the pupils had left."
Eton's half term lasted for four days, and then the school was closed for a further week to all those who were not taking exams.
The highest priority was given to pupils who were sitting their A2 papers.
Mr Wynn said: "All the boys were given Tamiflu, which was organised by the Health Protection Agency, and so the risk of infection was greatly reduced.
"The illness was very mild and in practically all cases it was short- lived."
The number of swine flu cases in the UK has now topped 500, but Professor Hugh Pennington, a bacteriologist at Aberdeen University, has said it would be reasonable to estimate that there are roughly twice as many cases as the official figure.
Teesside High School, a 370-pupil private school in Eaglescliffe near Stockton-on-Tees, closed on Monday after three Year 7 pupils were confirmed as having the virus.
But the school will stay open to those taking public exams. Pupils have all been invited to school to collect anti-viral medication.