More than three-quarters of England's state schools faced disruption due to today's walkout, the latest Government figures show.
According to the Department for Education, of 21,476 schools, 13,349 (62%) are believed to have closed to pupils entirely.
A further 2,951 (14%) were reported to be partly open, meaning 76% of England's schools were affected in total.
These figures include maintained schools, academies, free schools, university technical colleges and studio schools.
The DfE's figures show that just 3,351 state schools (16%) were fully open, while the rest (8%) are unknown.
Among maintained schools only, 63% were reported to be closed, with a further 13% partially open.
Of 1,449 academies, free schools, university technical colleges and studio schools, 57% were reported to be closed and a further 29% partially open.
David Cameron had earlier told MPs at Prime Minister's Questions that the "evidence would suggest that around 40% of schools are open".
Data gathered by the Press Association from 113 English local authorities suggests that in these areas alone around 11,500 schools have been affected in some way.
In Wales, more than 1,500 out of 1,776 schools have closed their doors, and only 30 of Scotland's 2,700 schools opened.
And in Northern Ireland roughly 800 schools, two thirds in total, faced disruption.
Overall, millions of children missed lessons today as their teachers took to picket lines.
Dr Mary Bousted, general secretary of The Association of Teachers and Lecturers (ATL), said: "No-one is pleased that schools are closed because we know what disruption that causes, but it's very important that the Government understands the depth of anger and hurt."
Speaking at a rally in Manchester, Dr Bousted said: "We are not here to say that there must be no changes. We are here to say to the Government, stop treating us with contempt.
"If you need to change public sector pensions because people are living longer, show us your sums. Make your case. And if you can do this, then negotiate properly. Stop engaging in megaphone diplomacy. Stop the threats and the misrepresentation. Start behaving responsibly. Start acting like a government not like a shower.
"And let's get a resolution to this dispute. We are willing to work towards a settlement. Is the Government willing to make the same commitment - not just by their words, which are cheap, but by their actions?"
Five teaching unions, collectively representing hundreds of thousands of teachers, schools leaders and lecturers, took part in today's public sector strike.
Three of them, the National Union of Teachers (NUT), the Association of Teachers and Lecturers (ATL) and the University and College Union (UCU) took part in astrike in June.
They have been joined today by the National Association of Head Teachers (NAHT) and NASUWT for the TUC's day of action.
The unions argue that the Government's public sector pension plans will leave teachers paying in more, working longer and receiving less when they retire.
Ministers argue that reform is needed to make pensions sustainable for the future.
Schools Minister Nick Gibb said today's strikes will "benefit no one".
"They will disrupt pupils' education; hugely inconvenience parents; and damage teachers' reputation," he said.
"It's irresponsible to strike while negotiations are ongoing. Many parents will struggle to understand why schools are closed when the pension deal on the table means that teachers will still be better rewarded than the vast majority of workers in the private sector."