By Sean Coughlan
BBC News education reporter
Christine Blower promises a long-running campaign over pay
The teachers' strike sends a warning shot to government that they are serious about a long campaign over pay, says union leader Christine Blower.
The acting head of the National Union of Teachers says school staff will not be forced into the "boom and bust" pattern of public sector pay.
Ms Blower has also not ruled out further strike ballots over pay.
About a quarter of schools in England and Wales seem set to be disrupted by Thursday's strike.
"It will make clear to government that we are very serious about this pay campaign - I hope it will make them realise that paying teachers below the rate of inflation cannot go on," said Ms Blower.
"What I say to parents is, yes, your children's education is being disrupted for a day, but the outcome that we're looking for is to make sure that there really are enough teachers - and that will only happen if pay is at the right level."
The last time that teachers felt angry enough to stage a national strike there was a Conservative government led by Margaret Thatcher.
And in the face of the current stand-off, Ms Blower says there have been no last-minute peace overtures from the Department for Children, Schools and Families.
Despite her radical reputation - she was labelled an extremist by a former NUT general secretary - she seems reluctant to adopt a confrontational stance.
"We've come to this strike action reluctantly, having tried other avenues for bringing to the government's attention that 2.45% is not acceptable."
But she is sensitive to the threat to teachers' image from the strike.
"I think there is a great danger that some people will seek to undermine the image of teaching - by handing out what might be considered an unwarranted vilification of teachers.
"But if you ask parents about the people who teach their own children, you will get a very positive response."
There has been criticism of the timing of the one-day strike, with warnings that it could interrupt the revision of pupils preparing for GCSEs and A-levels.
But the NUT leader defended the choice of date.
"We chose the date with extreme care. We chose a day on which there is only one public exam - an art exam - and we've given advice to members that this action is not about disrupting public exams," said Ms Blower.
The bigger picture, according to the union, is that a successful education system requires well-paid, well-motivated teachers.
And after a period of pay rises, Ms Blower says the government is being sent a strong message that teachers will not allow their pay to drift backwards again.
"We've been in the boom and now we're into the bust - and the government is looking to another three years of below-inflation pay increases.
"What we're saying to the government is, if you really do value teachers, then make sure that they're paid at least at the level of inflation - which we take to be the RPI, which is 4.1%."
Ms Blower says the union has no estimate how many schools will be closed by the strike - with the latest surveys suggesting that more than a quarter of schools will either be closed or partially closed.
Both the NUT and the government will be looking carefully to see the extent of the impact of this biggest classroom union flexing its muscles. It has been two decades since it was last attempted - and the NUT is acting alone, without the support of the other teachers' unions.
Once the dust has settled on this one-day strike, and presumably depending on how it has been supported, Ms Blower says that the union will consider the next step in its campaign over pay - leaving the option open for an escalation of action.
"What is definitely the case, is that we need to continue the campaign to make sure that pay levels are not eroded."