Some sociologists in Ohio are advocating for "a longer school year and more after-school programs to keep children active" because kids' body-mass indexes tend to jump up over the summer. That sounds like a good reason to stretch out the school year. An even better reason, though, is the skill loss kids suffer over the summer. Kids, especially poorer kids, lose something like 2-3 months worth of education time over their summer vacations.
There's also a lot of talk about after-school programs, but it seems to me it makes more sense to just make the school day longer. Setting up a whole new system to put the kids through seems much less efficient than just adding a couple of hours to the already-existing school day. This seems especially obvious if the after-school programs in question keep the kids on the school site anyway. And much like extending the school year, making the school day longer would be especially beneficial for poorer students, who are less likely to be supervised at home in the afternoons. In any case, if you're going to keep the kids on campus and teach them stuff, that sounds an awfully lot like "school", and I think it makes sense to treat it that way.
I'm not all about locking kids up in schools, though. I'd be willing to make a few concessions to students, including putting less of an emphasis on homework and creating a vacation system similar to those offered to working adults. Actually, I imagine those would be the sorts of concessions parents would be looking for, too, if they weren't already pleased to have somebody looking after their kids while they're at work.