It is an even greater shock that D.C. students’ learning has improved more rapidly over the past 15 years than that of students in 20 other urban districts whose performance we have assessed. What’s the reason for the shock? The fact that it’s the near-universal approach of the 13,000 public school districts in the United States to pay teachers on the basis of experience and extent of graduate education — not position or performance. This might not be so objectionable — except for the disquieting fact that teacher salaries then end up being virtually unrelated to effectiveness in the classroom