There is something that hasn't been mentioned before in this forum: not increasing the school hours, but increasing the school calendar.
Currently, the American school system is based upon an agrarian calendar, summer vacation is long, so kids can go help their parents work on the farm. However, this creates a big blank hole in the educational development of children. Studies have shown that kids lose about 2-4 months worth of education over the two-month summer vacation. If this time was shortened considerably, say to three weeks, they would lose far less then they currently are.
I say: increase class hours so that each class is about 1h 30m, instead of 45m, which is about average, and increase the length of a break-time between classes, instead of 5 minutes to rush from one class to the next, give them 15 minutes to actually decompress a little, then get to their next class. Also: make the summer vacation shorter, and all other vacations longer. 2 weeks during the winter, 2 weeks during the spring, and 3 weeks during the summer.
Also, I hate to say this, but increase the amount of homework. It sounds cruel, but practice really does make perfect. Every kid probably spends about 2 hours doing homework, and spends the rest of the time zonking out in front of the TV. Make it 3-4 hours, and really let the assignment sink in. This way, lessons do not have to be repeated ad nauseum.
The biggest problem is that people are saying we should teach better, not increase hours, but in reality learning is a 24 hour experience. A kid receiving 45 minutes of history isn't going to learn very much, and the half-hour of homework isn't going to help matters. A large problem with this is that most of school is rote memorization, of course it's boring, whereas college should be more about learning how to research.
People here are right, sometimes it's the teacher that makes a difference, and I couldn't agree more. My best teachers were the ones who made their subject interesting, who spoke about them with excitement and odd facts that help draw in their students. Unfortunately, not only has there been a dirth of accredited teachers entering the system, but the system itself has failed to correct itself on countless occasions, advising the removal of schools and teachers that do the worst, when, very likely, these are the ones who need the most help. The creation of pass/fail programs, rewarding the best and punishing the worst has only exacerbated the problem, forcing teachers to hammer into their students only the questions that will be on the test, rather than actually teaching them.