Labour is promising to cap class sizes at 30 pupils across all schools in England if it wins next Thursday's general election.
The pledge is an extension of the party's commitment in its manifesto to limit classes to this size at all primary schools.
The party said it would fulfil the pledge by recruiting nearly 20,000 extra teachers over five years.
The Conservatives said English schools were rising up international rankings.
The party has pledged an extra £7.1bn by 2022-23 for schools in England.
Labour's pledge to recruit nearly 20,000 teachers is similar to a promise made two weeks ago by the Liberal Democrats.
Lib Dem education spokeswoman Layla Moran accused the party of trying to "copy" them, but added that Labour had "no hope of meeting this target".
She said Labour would not be able to "square these promises" with leaving the EU if voters back the party's Brexit deal in its planned referendum, due to "thousands of EU teachers coming to work in schools each year".
'Things will move'
Labour said the recruitment would be funded from an extra £25bn in schools spending over the next three years. The party has also committed to ensuring all teachers have formal teaching qualifications within five years.
Shadow education secretary Angela Rayner told BBC Breakfast a Labour government could not make changes "overnight", but they would "reversing the trend" of increasing class sizes and lack of spending under the Conservatives.
"The investment would go in immediately, so the money that schools have had cut they would instantly see," she said.
"On 13 December, I can't bring in 20,000 teachers, of course not. But what I can do through our National Education Service [is] bring in the training and skills.
"So, things will move. Will it happen immediately over night? Of course not. But immediately from day one of me being education secretary we will put it in place."
There is a teacher shortage in England, with the latest official statistics showing a 15% shortfall in numbers beginning training for secondary schools.
But Labour's promise to recruit more may not be easy to deliver.
In subjects such as physics, fewer than half the required graduates have begun to train.
Schools hire teachers, not the government, and it is harder to attract and keep teachers to work in poorer areas.
The party also says it would cap all class sizes at 30. At the moment, just over 12% of secondary pupils are taught in classes with between 31-35 pupils.
In 2009, when Labour was last in government, it was just over 10%.
While slightly smaller class sizes could appeal to parents, the international evidence suggests it may make less of a difference to results than the quality of teaching.
Labour is also promising to train up 24,958 unqualified teachers. Official statistics suggest 98.7% of the teachers in England have at least a degree-level qualification, so who is the party talking about?
Some are trainees working towards becoming a qualified teacher. Some are teachers who qualified overseas. Others are described as instructors who bring a special skill from their previous working life.
Some are working in smaller units with excluded children or in schools for children with special needs.
But there have always been some teachers in these categories in England's schools.
The National Association of Head Teachers said 47,000 secondary teachers and 8,000 primary teachers would be needed by 2024 to keep pace with an expected increase in pupil numbers.
Its general secretary Paul Whiteman said: "We need significantly more recruits than Labour are suggesting just to meet rising demand, never mind reduce current class sizes."
"The new recruits we need will not magically appear, and nor will they stay if we don't also address the stress and unnecessary workload that is widespread in the system."
Pushed on whether Labour's pledges were enough, Ms Rayner said the party was being "realistic".
"We are promising a huge amount more than what the other parties are," she told Breakfast.
"Over the last seven years, the government has missed their recruitment and retention target. Under Labour, you would get 20,000 new teachers and 25,000 unqualified teachers getting qualified."
Schools minister Nick Gibb said that in government Labour would "would wreck the economy, leaving no money for public services".
He added that figures this week from the OECD's international school rankings showed English schools had risen up the league tables.
Mr Gibb added that schools in Wales, where schools are run by the Labour-led devolved government, were the lowest performing within the UK.
"Conservative education reforms are improving standards in our schools, meaning children can get a better start in life," he added.
Labour is also announcing new plans to tackle homelessness, including £100m a year for emergency winter shelters, £600m to build new hostels, and £200m to refurbish existing ones.
Shadow housing secretary John Healey told BBC Radio 4's Today programme that people living on the streets was not "inevitable", but a "direct result of decisions the government has taken in the past 10 years".
"Some of these things aren't political," he said. "It shames us all that we have people dying on our streets because they are homeless and shames Conservative ministers most of all.
"The tragedy is we know what needs to be done as we have done it before... It is a new moral mission for Britain."
A Conservative party spokesman said: "There is record investment going in to tackling homelessness - £1.2bn until April 2020 with a further £422m for 2020-21."
The Conservatives, like Labour, have pledged to end rough sleeping within five years if elected to government.