It started with financial delegation to schools when heads became responsible for their own budgets.
Staffing makes up most of the costs in schools and heads were unwilling to commit to a staff entirely made up of full-time permanent appointments.
They wanted a number of part-time and fixed-term appointments to provide flexibility, so if the number of pupils went down they could alter staffing and balance the books.
Very few schools now run completely on full-time teachers. It is important to get a good mix between full and part-time staff.
Heads also need to fill vacancies as best they can: if they cannot get a suitable full-time teacher then a part-time one may be appointed.
From this year, people with children under six will have the right to ask for flexible working hours. I think that will lead to more teachers with young children pressing for flexible working.
That may be good for schools as teachers who are parents would stay in the profession rather than leave, as quite a few do now. On the other hand there is a risk that small primary schools in particular, end up with just a head and deputy as full-time members of staff and all the other teachers working part-time. That would could lead to difficulties with timetabling and ensuring continuity for the children.
It is up to headteachers to make the decision but, by and large, many schools will accommodate one or two jobshares.
There are also quite a number of teachers leaving because of heavy workload who intend to become supply teachers. They feel that supply work lets them focus on teaching and do less of the other activities teachers have to engage in.
Maybe schools will create flexible hours for such teachers as a way of retaining staff.
I think the pressures of new legislation and the difficulties of retaining staff, combined with heads who always need some flexibility, will mean that the number of part-time teachers carries on going up for a few years yet.
But the situation will level off.
Alan Smithers is director of the centre for education and employment research, Liverpool University