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Low-achieving students often are taught by the least-qualified teachers. These disparities begin when teachers take their first jobs, and in urban areas they are worsened by teachers' sub- sequent decisions to transfer and quit. Such quits and transfers increase disparities in at least two ways. First, more qualified teachers are substantially more likely to leave schools hav- ing the lowest-achieving students. For example, of the new teachers hired in New York City's lowest-achieving schools in 1996-1998, 28 percent scored in the lowest quartile on the general-knowledge certification exam.' Of those remaining in the same schools five years later, 44 percent had scores in the lowest quartile. In contrast, 22 percent of the new teachers in the higher-achieving schools were in the lowest quartile, which only increased to 24 percent for those remaining after five years.2 Second, the generally high teacher turnover in lower- performing schools disadvantage students in those schools since the effectiveness of teachers increases over the first few years of their careers. Twenty-seven percent of first-year teachers in New York City's lower-performing schools do not return the following year, compared to 15 percent in the quartile of schools having the relatively highest student achievement.