Twenty years ago, school staffrooms were predictable places. Teachers appeared from Monday to Friday during term-time, taught their classes, maybe ran an after-school club and left with a pile of marking.
Staff numbers were roughly allied to the number of pupils, and working mothers of young children were about as common as sunshine in an English drought. Since then schools have been hit by tidal waves of initiatives, and yet the biggest structural change has happened quietly and almost unnoticed, except perhaps by the manufacturers of pigeon-holes. Teachers working part-time have roughly trebled in number.
In some schools, the increase has been a response to falling rolls and teachers' statutory non-contact time. But mostly it has been driven by the increasing trend for mothers with young families to return to work.
Numbers have grown through need and convenience, rather than through any official push or guidance: both a strength and a weakness, as local authorities interpret guidelines in different ways.
It cannot be fair that a 0.5 part-timer might be paid differently in, say, Birkenhead and Bognor, especially when full-timers are not. Some sit unpaid through assembly: others, through every staff meeting and inset day. Some get paid non-contact time: others do not.
Part-timers have been easy to exploit, either because family commitments mean they need to work shorter hours, or because that is all schools are offering.
Given the new Conservative leader David Cameron's agenda - worklife balance, progressive working conditions in the public sector and the need to make us happier - the Government's move to improve conditions for part-time teachers comes not a moment too soon.
Education Secretary Alan Johnson's letter to the School Teachers Review Body should end the exploitation of part-timers by ensuring they too get paid non-contact time. Less welcome is his request that the review body considers different pay arrangements for Wales.
But, overall, Mr Johnson has earned the gratitude of 80,000 people on whom the smooth running of our schools depends: not a bad way to start in a new job.