I am just about to start my 21st year teaching in the public school system. In that time I have had 5 different principals, and even more assistant principals. Without exception, it has been clear from the beginning that these admins primary goal was to further their careers, and improving the pedagogy in the building was secondary. As for the district level admins, they are politicians. They have no real accountability, other than "test scores" (which is about as bogus a measure of success as there is). Given that these scores are the default measure of the success or failure of a school's ability to provide a good education, teaching to the test is the focus of these admins, and many teachers who buy into the programs coming from the top. We are asked to focus our attention on those students who scored 2's on the test, in the hope that they can be moved up to 3's, which is passing. Those who score 1's aren't mentioned. In discussions with my last principal about how to support struggling students, he side-stepped the issue, finishing the conversation with, "They wouldn't pass the test anyway."
As Jim said, reduce class sizes is a huge goal for improving student learning. On a daily basis, I can't help Saul, who has processing delays and still keep borderline-genius Kelsey fully engaged due to her ADHD, while making sure the 30 other students in the room get what they need to gain the skills, knowledge and understanding of what we are studying,in a 50-minute period. It's an untenable situation.
Improving the lot of education in this country is a very complex issue, with many variables, but making those at the top as accountable as everyone seems to believe the teachers should be, and reducing the number of students in each class is where I'd start.