"We have regressed in that sense," said the journalist and TV presenter, as he visited St Thomas the Apostle College in Nunhead, as part of a pre-election project to engage young people in politics and democracy.
"There is no historical imperative that everyone in politics has to come from private school," he said.
And the narrowing of the social backgrounds of politicians was "one of the reasons our politics is less effective and less representative", said the editor of Private Eye.
When he was at university at Oxford, after attending an all-boys private school, he said there was a strong presence of grammar and comprehensive school students, running magazines and debating societies.
"It wasn't just loads of people from fee-paying schools, it was more of a parity, and I think that's been lost."
Such a deepening division, with the rich getting richer and the poor getting poorer, and "squeezing out the middle entirely", was a waste of potential talent, he told pupils.
"There's no point having a whole country full of young people if you don't give them the opportunities, if we say, 'Oh no, we only want 7% of the country to do anything - just you lot and not you lot.'
"There is a real problem with the poor not moving, being less socially mobile. It's very bad for any society."
Mr Hislop was visiting the Catholic secondary school as part of the Speakers for Schools project, in which people from a range of industries give talks in state schools, offering the kind of horizon-raising perspectives that might be taken for granted in the independent sector.
In a school hall question-and-answer session, Mr Hislop cast a critical eye over the education system.
"On the whole it has failed quite a lot of the population in terms of providing a first-class education.
"It's been endlessly reformed," he said, and "along the way a lot of the essentials were lost".
St Thomas the Apostle College has been rapidly improving, now rated as outstanding, and has adopted a strong emphasis on good behaviour and discipline.
"I'm interested in finding a school like this which has essentially seemed to me to have discovered some of the more traditional educational values.
"I appreciate people who have got jackets on, wear ties, understand the importance of discipline and the importance of education itself. These seem to have been lost a bit."
He also warned against "too much testing" in schools.
"You lose the bits round the sides, what education is about, the interesting bits, reading off the subject, engaging in things that aren't strictly, 'You must pass this at level seven.'
"I know why it happened - people thought we can't have people coming out of school who can't read or write, so that's all we're going to teach.
"But I think we have to be slightly more ambitious than that."
Seeming more moralist than satirist, he said schools needed "discipline, a desire to learn. If a school doesn't have an environment where people can learn, then it will fall apart".
He told the pupils, boys living in the Peckham area, that "there is absolutely no reason why you should not compete with anyone else in the country, irrespective of which school they've been to".
"That should be the point of secondary education."
In terms of faith schools, he said it was misleading for successful faith schools to be "lumped in" with other debates about extremism, pointing out that the so-called Trojan Horse problems in Birmingham were about mainstream state schools and not faith schools.
Faith schools had been a success in the "core business" of education, he said. "I think you should do whatever works - and you shouldn't demolish what works," he said.
One of the aims of Speakers for Schools is to get young people talking about politics ahead of the general election.
"If you don't vote, you can't complain," Mr Hislop told them.
In turn, they pressed him for his views on the topics that interested them.
He was against legalising cannabis, because the potential harm of stronger forms of the drug, such as skunk, were too much of an unknown quantity.
On immigration, he said it had been a subject that had been "ducked" for too long because of embarrassment about talking about it. It was a question of deciding on numbers and limits, in an open way.
And he seemed bemused by the lack of lessons in citizenship. "Sex education is compulsory, but being a citizen, who cares?"
He said Nick Clegg and the Lib Dems were still under the long shadow of ditching their promise on tuition fees, an issue still current with the schoolboys.
"It's very difficult to come back from that. It's quite a red line to cross," said Mr Hislop.
And he spoke of the importance of free speech and how he had known one of the victims of the Charlie Hebdo terror attack in Paris.
"It was a reminder that the price of freedom of expression can be very high."
But asked about intervening in Nigeria against Boko Haram, he said a decade ago it might have been considered, but now there was no public appetite for such engagements overseas.
And was his own job of making fun of politics contributing to the cynicism about elections?
No, he argued. Politicians and a free press could still change things.
"I'm not cynical, I'm sceptical," he said.
The gap between those educated privately and by the state is getting wider and social mobility is weaker than 30 years ago, Ian Hislop has told an audience of south London teenagers.