Deputy prime minister Nick Clegg described the Globe Academy in Southwark as a "brilliant school" that is "really going places" during a visit to mark the launch of the Government's new social mobility strategy.
"Don't take no for answer," he said. "Don't let anyone pretend that certain jobs are for certain people and some other jobs are for other people.
"You have as much right to do the very top jobs in all the fields that you care about as anyone else."
He added: "If we want to have a fair society where everyone can get ahead, every single one of you should dream whatever dream you want and just go for it."
Introducing Mr Clegg, Globe Academy's principal Jason Baigent said: "I'm delighted to see the emphasis the Government is placing on driving up social mobility."
The Lib Dem leader took part in a mini careers fair, chatting with small groups of students about their future ambitions.
Other high profile figures taking part in the event included former BBC One controller Jay Hunt, now with Channel 4, and Royal Institute of British Architects president Ruth Reed.
Bermondsey and Old Southwark MP Simon Hughes, who is the Government's Advocate for Access to Education, also joined the event.
As part of the new social mobility initiative the entire cabinet has agreed to take part in Robert Peston's Speakers for Schools programme to bring high-profile guest speakers to state schools. Last week the BBC's Huw Edwards visited SE1's Walworth Academy as part of the same scheme.