First, full disclosure: I'm not a teacher (yet), and I also don't have any personal experience with the school system in the US. I'm Canadian, living in Finland to study, but I've always had an interest in education and intend to pursue teaching once I return to Canada.
Now, ideas from Finland that I think should be considered:
Free lunch for EVERY student. So everyone eats the same food, together, at the same time. Teachers also eat for free if they sit in the cafeteria with the students. Lunches here are reasonably healthy (some form of meat/fish/protein + potatoes/pasta/bread + cooked veggies + fresh salad bar + milk) and I know no one really talks about it as a reason for Finnish students' success but I'm convinced that it helps.
Basically no such thing as private schools. Every kid goes to the public schools -> motivation for them to invest time and money into keeping the quality high.
This gets thrown around a fair bit in articles about why the education system here is so great, but I do think that more (ideally outdoor) playtime for elementary students is a good thing, and also pretty easy to implement (at least compared to many of the other ideas). They have a 45min/15min schedule of class time/recess here.
Finally, this is also frequently cited as a reason why the education system here is so great, but it's probably the hardest thing to change: respect for the teachers. Teachers are highly qualified and well-respected professionals, and are given a lot of freedom to teach as they like, and also a lot of downtime (i.e. good work-life balance). This helps to make teaching a desirable profession, thus attracting intelligent and motivated people to become teachers. Freakonomics did a pretty interesting story about this a while ago, worth a listen if you're interested.
Of course, Finland may not be the perfect comparison, since it's a fairly homogeneous country of only 5.5 million people. Taxes are quite high, and "social classes" are much less distinct. I know it's not really feasible to implement much of what I said, at least not in the immediate future, but I still think these are important discussions to have.
I'm really interested in what others have to say as well.