Class size has one of the largest impacts on quality of education.
Actually, no. There is a wide spread belief that class size improves education, but there is shockingly little evidence to support that belief. The biggest controlled study was the STAR Study done in Tennessee during the 1980s. It found the benefits to be minimal and uneven. Other studies have generally found even less benefit. Kids in early grades benefit most, with little to no benefit from smaller classes beyond grade 3. Most of the improvement goes to the underperformers. In some cases, the smarter kids actually do worse with small classes. This may be because they are forced to follow along with the class instead of reading ahead or learning on their own.
Much of the benefit from "smaller classes" may actually be from "quieter classes". Many young children have difficulty filtering out distracting noise. Good sound proofing, and reduction in disruptive behavior, can often bring as much benefit as smaller classes. Interesting, improving student/teacher ratios by adding teaching assistants has been found to provide no benefit.
If correctly targeted, smaller classes have their place, but they are far from a panacea.
Smaller classes with careless teachers are a waste, for sure. But I suspect that the lack of results regarding smaller classes is because lack of teacher's competence and willingness..
In other cases, the effort goes into the bottom students (the slogan "no child left behind" meaning really "no bright child is allowed to advance at a better pace than the mediocres", leaving no impact in the gifted and brightests.
However, no matter how many studies (fail to find such studies with clear results) supposedly exist, the issue is plain common sense, in a smaller class, the teacher has more time to nurture students in a customized way..
If you get smaller classes, you have to implement policies to ensure that the teacher will use such time advantage, to really improve customized teaching, and not focusing only in the ones with problems, but also in the brilliant students.