This article makes the common mistake of confusing correlation with causation. Some data points might correlate but that doesn't imply they cause each other.
Exactly right. Did they actually compare students from same school districts but different income families?
I would think that what is most likely is that good income family students (who can afford summer schools) probably go to better school, which is by itself self-reinforcing.
I don't think you'll get that kind of in-depth analysis and research from a "a nonprofit that promotes expanding learning time to improve student achievement".
I would think that what is most likely is that good income family students (who can afford summer schools) probably go to better school, which is by itself self-reinforcing.