What is it?
The belief that smaller class sizes mean better results for pupils. It's an idea that has huge support among parents, who (understandably) follow the logic that if there are fewer pupils in class, their child will get more attention and be more likely to succeed.
The public are quick to jump on the bandwagon too. In his book, David and Goliath: underdogs, misfits and the art of battling giants, Malcolm Gladwell notes that 77% of Americans would rather tax dollars were spent on reducing class sizes than raising teachers' salaries. Many teachers also back the concept, citing the more productive working environment of less crowded classrooms.
Now the cause has been picked up by the Labour Party, who pledged in their manifesto to reduce class sizes to under 30 for all five-, six- and seven-year-olds, after leader Jeremy Corbyn said students were "crammed in like sardines". But is it really a silver bullet for achievement?
What do supporters say?
In a nutshell: if there are less children to teach, more teaching can take place. Proposed benefits include more one-to-one feedback between teachers and students; less likelihood of challenging behaviour; more participation from students and a better sense of community between classmates.
A study from the Institute of Education found that smaller class sizes can be particularly beneficial for low-attaining pupils, whose engagement may be lower in large groups.
Research also suggests that class size can have a dramatic impact on achievement for all learners, but the numbers have to drop pretty dramatically. According to the Education Endowment Foundation, if a class can be reduced to "under 20 or even below 15", a greater effect can be seen.
This is because such a drop allows teachers to change their practice. The paper says: "The key issue appears to be whether the reduction is large enough to permit the teacher to change their teaching approach when working with a smaller class and whether, as a result, the pupils change their learning behaviours. If no change occurs, then – perhaps unsurprisingly – learning is unlikely to improve."
What do critics say?
Usually that it is simply too expensive for the returns it offers. John Hattie's seminal meta analysis of education research, Visible Learning, found that class size has a limited impact on student outcomes, despite being one of the more costly interventions.
He told The Telegraph: "[It] does increase achievement but the effect is very small. The change is small because teachers don't change their teaching when the size changes… Instead, parents should be comparing class size with teachers' expertise."
Researcher Eric Hanushek makes a similar argument in The Class Size Debate, where he points out that the concept has been widely discredited and should be pushed aside in favour of proven strategies, such as better professional development.
Meanwhile, Andreas Schleicher, director of education and skills for the Organisation for Economic Cooperation and Development, argues that large class sizes may be a necessary evil in a cash-strapped system. "If you have a certain amount of money, that's the sort of trade-off you have to make," he says.
"Smaller classes limit your capacity to pay your teachers well. If you have a limited budget that's basically your choice," he adds. "In upper secondary education it's what we see in high-performing countries like Japan, Singapore, and Korea. All of those countries prioritise teachers and teaching over infrastructure and class size."
What now?
With the UK's education system already under huge strain, it's unlikely that a cash boost to reduce class sizes to below 20 is on the horizon – particularly as this would require redesigning classrooms too. So while class sizes may fluctuate a little, the evidence suggests that schools would be better off investing in other methods to boost attainment.
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