More than 300 schools have closed fully or partially as members of Welsh teaching union Ucac strike over plans to change pensions.
The union, which represents about 5,000 staff, said about 89% of those who took part in a ballot voted for Wednesday's 24-hour strike.
The union claims some further education colleges will also be affected.
The UK government said it wanted "genuine engagement" with unions, and the action was extremely disappointing.
The union said the UK government's plans would leave members paying more and working longer.
Elaine Edwards, Ucac's general secretary, said: "The government's proposals are totally unnecessary and absolutely unfair.
"Asking teachers and lecturers to pay more each month, to work until they're 68 and even then to receive smaller payments in retirement shows a lack of understanding of the nature of the profession and also a complete lack of respect to educators."
She added: "The problem is the enormous deficit created by the bankers - but the government won't own up to that.
"For teachers, taking strike action doesn't come easily.
"We're all too aware of the effect on pupils and their parents.
"But we're concerned that this attack on pensions will have a negative impact on educational standards by making teaching a less attractive profession - and we know that that's a matter of concern for parents too."
Ucac estimates up to 3,500 people are on strike, across 620 institutions. These are mostly schools, but include some further education colleges and universities.
Phil Higginson of Ucac in Pembrokeshire said teachers felt strongly that it was a matter of principle that the government has "broken the terms and conditions of our contracts".
A UK government spokesman said: "We are totally committed to genuine engagement with the unions, we have a lot to talk about and there are proposals on the table for discussion.
"Central discussions have been going on for several months, and the government is committed to working with the unions to achieve necessary reforms.
"It is extremely disappointing that a small number of teachers from the Ucac union are planning to lose a day's pay and go on strike while serious talks are still ongoing.
"This is a genuine and meaningful dialogue, which includes discussions about how to implement the changes on contributions set out in the Spending Review.
"The government is committed to this dialogue in order to agree a way forward. However, the unions also need to commit to genuine engagement and make constructive proposals."
One business in Cardiff stepped in to help parents struggling to find childcare.
Gwilym Boore, the director of Goal Football Centres, who put on "emergency" training sessions, said it helped the company and its customers.