In a letter posted to parents on its website, the independent school said the girl was "at home and well".
South Hampstead is the third UK school to close since the outbreak, following Downend School in South Gloucestershire and Paignton Community and Sports College in Devon.
However, the Health Protection Agency (HPA) said the school attended by the 11-year-old will remain open as the child "did not attend school while symptomatic".
The Department of Health (DoH) said the child had recently returned from the US.
Tourist Eddie Sweeney has been confined to Hong Kong's Metropark hotel after a guest became ill
Several Britons are still under quarantine in a Hong Kong hotel after a guest there became China's first confirmed case of swine flu.
Eddie Sweeney and his wife Terry, from Buckinghamshire, checked into the Metropark Hotel on Thursday for two days but were told on Friday they and about 300 other guests would be confined to the building for a week.
Mr Sweeney told BBC News: "Effectively we're in our rooms, apart from going to collect meals. Some of us are using the stairs as a way of exercise. Otherwise we're confined.
"It's very frustrating. We were on our way home - I think it would probably have been worse if we were on our way out - but we do accept the fact that this is the way the Chinese do it.
"I have to say they've been very efficient, polite and helpful at all times."
Scottish Health Secretary Nicola Sturgeon said there was one further probable swine flu case in Scotland, in addition to the four already confirmed.
It involves a person from Fife who travelled from Las Vegas to Edinburgh via London on 30 April.
She said eight of the 10 people regarded as close contacts of the man from Ayrshire on flight BE7096 on 30 April had been traced, none of whom were showing any symptoms.
He is thought to have picked up swine flu while in Texas and was diagnosed with the virus on Sunday.
The number of cases in London stands at six, while other cases have been confirmed in Newcastle, Oxfordshire, Devon, Merseyside, Worcestershire, Gloucestershire, and Central Scotland.
The six-year-old girl in Oxfordshire has made a full recovery, doctors have told the BBC.
Health chiefs are awaiting the results of more than 700 further tests.
Mr Benn told GMTV swine flu was so far "relatively mild and responds well to treatment here in the UK so far but we have got to remain vigilant".
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