Strike disruption has hit thousands of schools across the country as public sector workers staged a walkout over pensions.
Millions of children remained at home as schools were forced to close their doors completely, or remain open for just a few pupils.
Early Government figures suggest that almost three quarters of England's state schools were hit.
And data gathered by the Press Association from 76 English local authorities suggests that in these areas alone almost 8,500 schools have been affected in some way. In Wales, more than 1,500 out of 1,776 schools have closed their doors.
Dr Mary Bousted, general secretary of The Association of Teachers and Lecturers (ATL), said the union estimated that 90% of schools would be affected.
Many headteachers will not have informed government of whether they intended to close their school or not, she said.
Dr Bousted 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."
The Department for Education (DfE) said it believes that more than half of England's 21,700 state schools (58%) are closed, with a further 13% partially shut.
Around 13% are open, the DfE said, while the rest are unknown. Schools in Scotland, Wales and Northern Ireland are also affected.