In a field of revolving-door leaders, Thomas W. Payzant is the exception to the rule. Amid the rise and fall of superintendents in city after city, he has led the Boston Public Schools for nearly 11 years. Now, as he prepares to retire, the district offers a rare glimpse of what is possible with stable leadership in urban education. It is a story with many strands, including new curricula, data systems, and teaching methods. A critical ingredient has been staff training woven into the fabric of teachers' jobs. The aim has been to change the way educators think and go about their work. During Mr. Payzant's tenure, the Boston schools have generally matched or bettered statewide gains in student test scores. The district is the only system in the country to have been named a finalist five years running for the Los Angeles-based Broad Foundation's annual prize honoring improvement in urban districts.