[dohko]

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.

[tomkit]

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".

[pytrin]

It's easier to just run a couple of surveys and pick the stats that make your case stick.