is the reform that parents want most and that evidence supports least clearly.
Large studies find effects that are real, small and expensive per unit of improvement.
Reducing a class from thirty to twenty-five changes very little on its own.
Reducing it to fifteen changes a great deal and costs roughly twice as much to .
The effect is also in the earliest years and among the poorest pupils.
A general reduction across all years therefore spends most of the money where it does least.
The matters more than the number and is rarely discussed.
A smaller class helps only if the teacher teaches differently in it.
work finds that many teachers deliver the same lesson to twenty as to thirty-five.
Where training the reduction, the effect roughly doubled.
There is a further cost that appears only at a national scale.
Halving class size doubles the number of teachers required, and the additional teachers are, on average, less experienced.
A reform that lowers the average quality of teaching while lowering class size can produce no gain at all.
Countries with the strongest results have generally chosen larger classes and better paid, better trained teachers.
That is unpopular with everybody except the people who have measured it.