Having smaller class sizes feels like common sense—better for kids, better for teachers. And some rigorous research suggests that through third grade, dramatically smaller classes of 13 to 17 students positively affects student learning for any given teacher.

Unfortunately, significantly lowering class sizes removes students from the best teachers’ classrooms and necessitates hiring significantly more teachers, meaning schools must dig deep into their applicant pool. This may be why large-scale implementation of class-size reductions has not increased student learning as expected. As Bryan and Emily Ayscue Hassel have written:

“A large-scale reduction requires hiring massively more teachers, dipping deeper and deeper into the applicant pool. It also reduces the number of students who have excellent teachers—the ones who produce more than a year’s worth of student growth each year, necessary to close proficiency gaps and help students leap ahead.

Suppose K–5 Elm Street Elementary has 100 kids per grade. If it has class sizes of 25, it needs 24 teachers. To get to class sizes of 17—what it takes to get the benefits cited above—the school needs 36 teachers, or 50 percent more. If the school’s district needed to hire 300 teachers per year before, it needs to hire 450 now. So, it gets the 300 teachers it would have hired…and then dips 150 ranks lower into its applicant pool for the rest.

Ouch. Double ouch for hard-to-staff rural and urban schools that could not fill their teaching slots previously. Triple ouch for districts if the mandate isn’t fully funded by the state—they must scramble to pay for a reform that won’t put an extra dime in any current teacher’s pocket. Ouch, ouch, ouch.

In addition, if Elm Street is typical, it probably has about six excellent teachers among the 24. Those six teachers teach 25 students each, reaching 150 children total with terrific instruction. Under the new policy, they teach only 17 students each, for a total of 102.“

That’s tough anytime, but even more so if schools are facing growing teacher shortages.