> It's a pretty well-established fact that children of two-parent families have significantly better outcomes than those of single-parent families

Does there being children from single-parent families in the same school as your child lead to your child having significantly worse outcomes?

Recall the context from the article:

>> For example: an American family in 1975 could send their children to public school on the assumption that the vast majority of other children would belong to intact families, communities like their own, and would speak English as a first language. Now, realistically, many parents must turn to private schooling for the same reassurances.

It is listing things about other children that it implies drag down your child and so leads parents to turn to private schooling.