Teachers have welcomed the introduction of an agreement allowing them more time to plan lessons.
Jacqui Smith has said the deal is essential and 'not optional'
From Thursday, they will be entitled to spend 10% of their working week on planning, preparation and assessment of pupils' work.
General Secretary of the NASUWT, Chris Keates, described the changes as an "historic development".
The PPA deal follows earlier agreements to release teachers from administrative tasks and limit cover teaching.
Schools' minister Jacqui Smith said the change, which applies to England and Wales, would raise standards in schools.
Fewer than 15 schools in England have said they would have difficulty implementing the agreement, the Department for Education and Skills (DfES) said.
Ms Smith said 23,000 schools, over 99%, have confirmed they are ready to go ahead and almost 8,000 of these had already put planning time in their teaching schedules.
More money had been made available to schools to fund the provision of PPA time, she added.
"In 2005/06, school funding will reach over £4,000 per pupil, an increase of over £1,000 in real terms since 1997."
NASUWT head Ms Keates was upbeat on the changes.
"The provision of guaranteed time for planning, preparation and assessment - which cannot be used for anything else - will enhance the quality of lessons and enable teachers to continue to raise standards," she said.
"It emphasises the crucial importance of a teacher's core role - teaching.
With other changes since 2003, she said, PPA would help teachers take a more personal approach, reduce pressure and their workload.
'Assist not replace'
The Professional Association of Teachers said fears of unqualified staff taking over teachers' work were unfounded.
"This is not what PPA is about," said principal professional officer Deborah Simpson.
"Teaching assistants will assist, not replace, teachers.
"Support staff with the appropriate qualifications, training and experience will enhance pupils' learning."
Leader of the Secondary Heads Association Dr John Dunford also welcomed the deal, and said children would reap the benefits of a better-structured working week for teachers.
But some head teachers have said they cannot afford to cover teachers' planning time and the National Association of Headteachers voted to pull out of the agreement in March.
The deal entitles teachers to a minimum of 10% planning time
NAHT national pay and conditions officer Kerry George said although most schools would be able to implement the deal, many head teachers felt the agreement was unsustainable.
"Most schools will deliver it, but it will be at the expense of worse conditions for heads, as well as deputy and assistant heads," she said.
"Schools have no confidence that resources will allow them to deliver it next year."
Primary schools in particular have expressed concern that they will be forced to consider redundancies to cover the cost.
Chris Williams, head teacher of a primary school in south Gloucestershire, in May told the BBC News website he would rather break the law than sink into debt.
But Steve Sinnott, NUT General Secretary, has welcomed the introduction of planning time, as long as the government ensures every school has sufficient teachers to implement it.
"The quality of education received by pupils must not be diminished by the use of people who are not qualified teachers undertaking teaching work in the classroom," he said.
"Parents must be alert to the possibility of schools using unqualified people in this way."
Unison, the union which represents many school support staff, welcomed a greater role in delivering education.
Head of education services Christina McAnea said PPA: "Gives schools the opportunity to use our members' knowledge, skills and experience."
But she pressed for support staff covering PPA time to be paid at the correct level.