A new study of 35 charter schools in poor neighborhoods of New York City, which use a variety of approaches, shows that some things people think make schools better don't, including smaller class sizes and more teachers with advanced degrees. The things that do raise student test scores include more feedback for teachers, adjusting instruction methods based on data about what is working, and intensive tutoring for students having trouble. The researchers also emphasized the culture of the school, finding that an emphasis on academic excellence and a "no excuses" approach helped raise scores. Trying to raise self esteem has no effect.
Abstract here; summary here.