Billions of pounds have been pledged to England’s schools after years of lobbying by heads and teachers for more cash.
Money will be invested in primary and secondary schools over the next three years, the government has announced.
School leaders welcomed the move but warned the "crisis is now" and extra funding is needed as soon as possible.
Mr Johnson told ITV News Political Correspondent Libby Wiener: "What we're doing now is making sure that we level up across the country and we're putting £14 billion and the objective is to ensure that every school gets an increase.
"The schools that have fallen furthest behind will see the biggest uplift."
"People will say 'well, where's the money coming from?' Actually, the cash is there for now, for the next three years, £14 billion over three years.
When asked if he'd found a magic money tree to pay for the funding, the PM said: "No, the formula is basic, modern, conservatism: believing in the market economy, believing in enterprise, supporting wealth creators.
He added how Chancellor Sajid Javid believes the cash boost will still be affordable even if the UK leaves the EU without a deal.
"Saj is absolutely confident that we can do this within the headroom that we've got."
The overall schools budget is due to rise to £52.2 billion in 2022/23, the government said.
It is understood the current schools budget stands at about £45 billion.
The schools investment will be staggered, with much of the money coming at the end of the three-year period.
In 2020/21, schools will get a £2.6 billion rise, £4.8 billion in 2021/22 and £7.1 billion in 2022/23.
The money covers real-terms rises in school budgets due to factors such as inflation, increases in the pupil population as well as additional extra funding.
In addition to the funding pot, £1.5 billion each year will be put into teachers’ pensions.
The Government said this means that the overall three-year funding package totals £18.9 billion.
It is understood discussions around teachers’ pay are still ongoing.
Speaking to a group of children at Number 10 earlier, Mr Johnson said the investment "is to make sure that all schools in our wonderful country get more money, but particularly the schools that have fallen behind in their funding."
"Teachers change lives, they certainly changed my life and I hope they will continue to do a fantastic job in our country."
Geoff Barton, general secretary of the Association of School and College Leaders (ASCL), said: "We are pleased on behalf of schools and students that the government has listened to our repeated warnings about the scale and severity of the funding crisis, and has committed to desperately-needed additional funding for education over the next three years.
"We will be studying the figures in detail as soon as this information is available to understand exactly what the additional funding covers and how this commitment will be implemented. The crisis is now and extra funding is needed as soon as possible."
He added: "This announcement comes at a time of great political uncertainty and the Chancellor intends only to set departmental budgets for 2020/21 in his spending round announcement next week.
"This will raise questions about what will happen in the event of a no-deal Brexit and the possibility of an early general election.
"Whatever the circumstances over the next few years, it must be a national priority to deliver the additional funding which has been pledged today."
School leaders and teachers have led a long-running campaign calling for more investment, warning that there is a severe shortfall in funding.
Previous analysis by the Institute for Fiscal Studies (IFS) suggested that schools have faced budget cuts of eight per cent since 2010.
Education Secretary Gavin Williamson said: "What we’re going to be seeing is every school in this country is going to be having increases in funding and those schools that have historically had lower levels of funding are obviously going to be catching up with that and making a step towards delivering on the Prime Minister’s pledge for every school to be able to be receiving - whether it’s a secondary school, a minimum of £5,000 per pupil, or a primary school at £4,000 per pupil."
Paul Whiteman, general secretary of the National Association of Head Teachers (NAHT), said: "After five years of disappointment on funding, I’m sure the Prime Minister would forgive us only giving this a cautious welcome whilst we await the full details to come from the Chancellor’s statement on Wednesday."
He added: "On the face of it, this investment appears to repair some of damage that has been done to the education system since the cuts began in 2010.
"We’ve won the argument that it was only going to be new money from the Treasury that would solve the school funding crisis. More money is welcome.
"There’s no extra money for schools this year, so that’s still a big problem for schools whose budgets are already at breaking point."
Angela Rayner, shadow education secretary, said: “This comes nowhere close to meeting the Prime Minister’s pledge to reverse the Tories’ education cuts, let alone matching Labour’s plans to invest in a National Education Service.
"Instead, it is yet another con trick by a politician who has shown time and again that you just can’t trust his promises."