We've all heard the commonplace that as far as class size goes, bigger is not better.

In smaller classes, students benefit from more individual attention from the teacher. The teacher isn't all strung out trying to keep tabs on thirty little glue-sniffers. Parents get to think that their kids will have better test scores and a sense of community. The school board feels more cost-effective because it gets higher graduation rates. Win-win-win-win.

But don't just buy all those wins without looking at the way the game is played. Some studies have made the case that class size matters more in some situations than others: in particular, kids are understood to need more attention in early grades, and traditionally disadvantaged groups have also been shown to benefit from added attention (with English-language acquisition, for example).

So what is a normal class size, then?

The general vibe is that 22:1 for grades K-3 is too big, and over 30 is a stretch for fourth grade onward.

So we're saying 21 six-year-olds is a piece of cake? Um, no. Groups of 14-16 are generally more effective: not so big that the teacher has his or hands totally overflowing, but big enough for there to be plenty of opportunities for group activities, playing and learning in teams or pairs, and chances for students to learn from each other.

So "the smaller the better" also ain't the case.

Why is it important to get the right number in a class? Like we said, all that stuff about teachers getting to pay more attention to students and test scores on the up and up. But there's more.

One Tennessee study shows that class-size reduction works because students change their behavior: students in a smaller class are less able to hide in back or make faces when the teacher's not looking. Plus, they potential troublemakers benefit by working more closely with classmates. There are those win-wins again.

Still, it's not as simple as shrinking the ratio. While most sources will tell you that smaller class sizes are generally more effective, it's worth noting that even if you're lucky enough to get 16 tops year in and year out, if the quality of teaching or school leadership is lacking, it's not going to do you much good (source).

Not to mention all those other factors like school resources, parent involvement, size of the school, whether the kids like the color the walls are painted…the list could go on.

Plus, reducing class sizes only really makes a difference only if teachers have the training and administrative support to match their methods of teaching and interacting with both students and parents to the size of the class, and the real-live students themselves ( source).

So don't take a ratio at face value. First, do your research and figuring out what you've got to count on.