I think it does. Increasing the amount of time that kids spend in school is a promising strategy for improving kids' academic achievement. The literature on summer learning gaps provides insight into why this could work.
Here's how researchers have gone about studying summer learning gaps. Imagine we give a kindergarten student a test at the beginning and end of kindergarten, and then again at the beginning and end of first grade. We can now calculate a rate of learning during the school year, as well as a rate of learning during the summer. Since the early 1970s, we've known that poor kids learn at a much lower rate during the summer than their advantaged peers.
This adds up over time. Karl Alexander found that two-thirds of the reading achievement gap between poor and rich 9th graders in Baltimore is explained by how much they learned during their elementary school summers. (Links for more info: Karl Alexander's recent work - written up here in Ed Week - or Doug Downey and colleagues paper using data from the Early Childhood Longitudinal Study. )
During the school year, every night from 3PM-11PM is a mini-summer. Advantaged kids participate in tutoring, art, music, and sports when school lets out, but disadvantaged kids don't have access to activities that keep them learning through the evening. While it is an expensive intervention, keeping poor kids in school longer - both during the school year and during the summer - is a policy option that deserves more attention.
Enjoy the weekend, everyone!