Field Elementary in Columbia offers optional single-sex classes, as does Carmen Trails Elementary in St. Louis county. But, unlike Carmen Trails, which added a grade to the program this year in response to parents’ demand, Field has no intention of expanding without proof that single-sex classes raise test scores.

Field will never get the proof it wants. To show that single-sex schooling improves student achievement, you would have to randomly assign some students to single-sex classes and others to coed classes. Otherwise, you can’t tell whether the students in the single-sex track are comparable to their peers in the coed track. Perhaps students who struggle are more likely to look for a change and to opt in to the single-sex classes. Or, it could be that the most involved parents seek out single-sex education, and that parental involvement gave those students an advantage. It’s impossible to sort out these factors as long as the program remains voluntary.