D.C. Public Schools has some chronic problems keeping good principals around. Over 250 principals have turned over in DCPS in the last ten years, and as research consistently shows that effective principals are characteristic of effective schools, it’s time D.C. start recruiting, training, and (perhaps most importantly) retaining strong school leaders. Over the weekend, the Post covered Schools Chancellor Michelle Rhee’s nationwide recruitment efforts to attract new principals to D.C. schools. The District has launched…