The exclusive London event, in front of a crowd of around 2,700 people, is Ms Obama's only stop outside her 13-day USA roadshow.
Obama said her visit to the London school in 2009 inspired her to use her time in the White House to campaign for education for all girls as well as work to combat childhood obesity. "It ain't equal. I tell women that whole "you can have it all"-mmm, nope, not at the same time, that's a lie".
"It's not always enough to lean in because that s-- doesn't work all the time", she reportedly said.
Obama recalled many protocol briefings before meeting Queen Elizabeth during a state visit, when the latter, driving her auto, picked them up from their helicopter at Windsor and told them: "Sit wherever ... it's all rubbish, just get in". "It is hard", she said according to Vanity Fair. All right, I'm back now. She then repeated her assessment, sans the profanity.
She said: "Why is that?"
"So oftentimes it's not equal".
The first African-American first lady discussed the importance of making elite universities feel attainable to young, working class girls from Black Asian Minority Ethnic (BAME) background.
Waiting at the royal estate to drive them to lunch was Her Majesty and Prince Philip.
"Leaning in" was first popularized in 2013 by Sandberg, the now-embattled Facebook executive, in her book titled, "Lean In: Women, Work, and the Will to Lead". This is far from the first time she has brought up this point.
But some people took offense to Obama's unintentional cursing, calling her "classless".
On her career path and life before the White House: "I've learnt to grasp the art of reinvention. Never underestimate the power of what we wear to communicate another layer of who we are and what we want to say". "Finally", tweeted one person.
"I have no intention of running for office, ever", she wrote.
Only for him to reply "now that ought to shut you up;" and she admits it did!
She also spoke of her marriage and how the former president treats her with love and respect, and had a few tips for the men who came to see her.
But that's exactly what Michelle Obama did during a public event in NY on the weekend.
The advice for women to lean in has been criticized before.
Obama was asked about the philosophy of "leaning in" for women who are finding it hard to balance their home life with their career life. She used an expletive. Today there are 11.
"Here's the secret. I've been at every powerful table that you can think of". Am I good enough? So it's interesting to me, when people ask me 'how did you know how to be the first lady?' as if all my history just disappeared because sometimes that's what happens when you become the spouse of - you lose yourself in the title of spouse. Part of it is having that aspiration and that goal. We are demonised, we are too loud, we are too everything.