By Mike Baker
BBC News education correspondent
It is one of the biggest changes to schools in England and Wales for many years.
Yet, as the new academic year begins, the government is hoping that few parents and pupils will notice that anything has changed.
From this week, for the first time ever, all teachers are entitled by law to 10% of the timetable free from teaching in the classroom so they can get on with what is called PPA, or "planning, preparation and assessment".
Put another way, this means that each teacher must be given the equivalent of half a day a week away from the classroom.
In primary schools, where teachers usually stay with their class throughout the day, this is a revolution.
It could also prove a nightmare for head teachers, especially in small schools where there is less flexibility of staffing.
They must make their timetables work, week in week out, without simply transferring all the pressure of work to classroom assistants.
The government is confident that almost all schools will manage this.
Some will use classroom assistants to supervise classes, others will double up classes for certain activities, and others will rely on sports coaches, music specialists, and outside speakers to take classes for short periods.
Teachers will become as adept at analysing charts and trends as City financiers and market traders
Yet as the old school year was ending, there were warnings that might have sent a shiver down ministerial spines.
They predicted that mixed-age classes would be doubled up for whole morning or afternoon sessions or that some pupils could even be sent home for part of the week.
One of the head teachers' unions, the NAHT, pulled out of the agreement to implement the reforms because, although they agreed with the principle, they feared the extra money to make the reforms work was inadequate.
They joined the largest teachers' union, the NUT, who had refused to sign the workload agreement from the start, because it failed to guarantee that no class would be taught by anyone other than a fully qualified teacher.
So ministers will be hoping that September passes uneventfully. Their worst nightmare would be head teachers sending pupils home or something disastrous happening to a class under the sole supervision of a classroom assistant or sports coach.
But why are they taking this risk? Why do ministers see the workload reforms as so important?
After all, the teacher shortage has eased. Indeed in primary schools there are now signs of a surplus of teachers in some parts of the country, with graduates from teacher training courses struggling to find jobs.
So, to be cynical, the lure of 10% time free during the school day, in order to reduce teachers' overall working week, is no longer needed to attract new entrants into primary school teaching.
No, teacher recruitment - or even sympathy for teachers' long hours - is not the primary reason for this change. It goes deeper than that.
The workforce reforms are seen by ministers as a prerequisite for a bigger change to the teaching and learning in schools. It goes by the name of "personalisation". This is the buzzword in government policy circles.
The "personalised curriculum" is one where teaching and programmes of study are tailored to suit each individual child.
It requires careful monitoring of each pupil's progress, week by week and term by term. It means collecting and analysing data of performance in reading, writing, maths and other subjects.
It means catch-up programmes for those who begin to fall behind.
In short, teachers will become as adept at analysing charts and trends as City financiers and market traders.
Teaching assistants will play a bigger role under the changes
It also requires that teachers have the time to check each child's progress, to diagnose their needs, and to devise the best teaching plan. This is why they need time out of the classroom.
They will also have to make this information more readily available to parents who, in turn, will be encouraged to demand the appropriate action plan for their child.
Yet that is not all. This "personalised curriculum" also requires a new form of teaching. It cannot work if the class teacher is standing at the front of the room all day long.
Instead the teacher becomes a facilitator. Like a bespoke tailor they measure, design and plan each child's educational suit. They will set work, monitor progress, and assess outcomes.
While they are doing all this, someone else has to be the constant presence in the classroom. Someone else has to prepare the classroom wall displays. Someone else has to collect the dinner money or do the photocopying.
Increasingly, that someone else will be a teaching or administrative assistant working under the teacher's supervision.
It is a potentially radical step-change to teaching; perhaps the biggest since the end of the monitorial system and the shift away from older pupils supervising the younger ones to the conventional primary school model of a single teacher alone in their own classroom.
What does it mean for teacher professionalism? The last two decades have seen a steady erosion of autonomy, as teachers have been required to deliver a nationalised curriculum and oversee nationalised tests.
Will the personalised curriculum give them back some autonomy? Will it enhance their professionalism by making them the lead professional, with a strategic role, in charge of a team of para-professional assistants?
Or will it bog them down even further into a morass of forms and performance data, leaving less time, space and energy for spontaneity and creativity?
This term marks a crucial period in the development of the teaching profession. The impact on children will be immense. Watch this space.