"I was surrounded by extraordinary women in my life who taught me about quiet strength and dignity," she told 100 children at the Elizabeth Garrett Anderson School in Islington, north London, where there is such a racial mix that the student body can speak 55 languages.
It was the First Lady's first solo visit since her arrival in London. She drew a crowd of about 300 cheering onlookers on the pavement outside the school.
She added: "You too can control your own destiny, please remember that. Whether you come from a council estate or a country estate, your success will be determined by your own confidence and fortitude.
"Although the circumstances of our lives may seem very disengaged, with me standing here as the First Lady of the United States of America and you just getting through school, I want you to know we have very much in common. For nothing in my life ever would have predicted that I would be standing here as the first African-American First Lady.
"I was not raised with wealth or resources or any social standing to speak of. I was raised on the South Side of Chicago – that's the real part of Chicago."
Since setting foot in London on her first visit outside North America in her new capacity as First Lady, Mrs Obama has drawn comparisons with Jackie Kennedy, despite the moment when Americans in particular recoiled in horror when she was seen to put her arm around the Queen.
It is believed, without supporting evidence, that touching the Queen is a breach of protocol bordering on high treason. If so, the Palace was very relaxed yesterday about the "crime". "This was a mutual and spontaneous display of affection and appreciation between the Queen and Michelle Obama," said a spokeswoman.
Mrs Obama's school visit was a highlight of a day's schedule that kept the wives away from the ExCeL centre, where their husbands were meeting. It included a visit to the Royal Opera House in Covent Garden for a programme of classical music and dance, and a private lunch.
In case the whole G20 experience had not been surreal enough already, they were also treated to a reading by J K Rowling from her latest fantasy, The Tales Of Beedle The Bard. The children's novelist had been a star guest at dinner in Downing Street the previous evening, when she was flattered to learn that Barack Obama has read her books aloud to his daughter, Malia. The US President, and Svetlana Medvedeva, the wife of the Russian President, both insisted on having her autograph.
As usual at these summits, there was more public interest in how the wives dressed than anything they said or did. Sarah Brown was the first to arrive at the Royal Opera House, in a snug-fitting royal blue Graeme Black outfit, dark tights and Russell and Bromley court shoes, and wearing a chain by Pippa Small.
Michelle Obama was in a blue, white and maroon diamond-patterned cardigan, topped off with sequins, a bright blue prom-style Jason Wu dress and a double-string pearl necklace. Others went for traditional attire, including Gursharan Kaur, the Indian Prime Minister's wife, who wore a sari.Reuse content