Pledges of cash have been rolling in since parents announced plans to attempt to raise 2 million within weeks to take ownership of St Margaret's School, which went into administration last week.
The six-figure donation was received yesterday but the identity of the benefactor has not been revealed.
Lisa Ahmed, who is vice-chair of the school's Parents and Friends Association, said: "We had a fairly substantial six-figure pledge by one person so we are looking at some quite considerable amounts which have been donated.
"The level of support is incredible. Most of the donations are from former pupils or former teachers or parents of former pupils."
The consortium of parents who have been working with receivers KPMG on their business plan to take over the running of the school – which would be a first for Scotland – were due to meet the rest of the school body last night to try to "galvanise" support.
With no clear future yet decided for the school and a looming closing date of 29 June, parents of the 397 pupils have been looking elsewhere to try to secure their children places after the summer holidays.
They have been in touch with some of Edinburgh's other leading private schools – including St George's School for Girls, the Mary Erskine School and George Watson's College – while many have been requesting places at council-run schools.
Almost one on ten of pupils currently attending St Margaret's have secured places in the city's state schools, while the city council's education department is still going through requests.
So far, 28 primary and five secondary pupils have been offered places in some of the most popular council-run schools in the south of the city.
Mrs Ahmed, who has two children at St Margaret's and one who was due to start after summer, warned that a solution needs to be reached soon
"We need to try to galvanise everybody and assure them with some firm figures."
Councillor Marilyne MacLaren, city education leader, said: "We know that some parents may need our support and we have been very pro-active."