Collective bargaining is common in American public education, but its consequences are poorly understood. We focus here on key contractual provisions--seniority-based transfer rights--that affect teacher assignments, and we show that these transfer rights operate to burden disadvantaged schools with higher percentages of inexperienced teachers. We also show that this impact is conditional: It is substantial in large districts, where decisions are likely to follow rules, but it is virtually zero in small districts, where decisions tend to be less formal and undesirable outcomes can more easily be avoided. The negative consequences are thus concentrated on precisely those districts and schools--large districts, high-minority schools--that have been the nation's worst performers and the most difficult to improve.