My favorite statistic from recent education research is simply this: from Hoxby's research on New York charters, the characteristic of a charter school most closely associated with high achievement (as measured by test scores) is "Mission statement that emphasizes academic performance." This is a more statistically significant increase than those from longer school day or year, different hiring and compensation, any specific curriculum, class size or other structural changes. Chew on that for a while.
Keep it in mind when pondering the general question of why charter schools perform no better on average nationwide than traditional public schools. The history of charter schools parallels the standards and accountability movement, but it is not of that movement. Many charters have missions not exclusively focused on academics, particularly as measured by standardized tests. This was regarded as a feature, not a bug, and even this spring no less than the RI Board of Regents approved a new charter whose mission has no particular focus on academic achievement.
If the charter movement as a whole is going to change these aggregate stats, they're going to have to purge the schools that lack a singular focus on achievement as measured by test scores, graduation rates and other placement stats. In fact, I'm getting the feeling that process is already starting. Whether that reflects the spirit of community initiative and innovation that launched the charter idea is another question.