The primary purpose of this paper is to present evidence of significant differences in the effectiveness of schools serving low-income children; a secondary purpose is to show that one reason why more learning takes place in some classrooms and schools than in others has to do with differences in the effectiveness of teachers due to teaching experience and "vintage effects." Three samples of black second and third graders in New Haven were tested in reading and math at the beginning and end of the school year. Significant differences were found in the amount of learning taking place in different schools and different classrooms, even after taking into account the skill levels and family background each child brought to school. Findings also showed that, in all three samples, teachers with 3 to 5 years experience were more effective than teachers with less experience. Additional experience beyond the 5 years, however, did not increase effectiveness but reduced it slightly. Further investigation of the topic in a large midwestern city showed this apparent decline to be a research artifact produced by "vintage effects" (i.e., having to do with the lower quality of teacher hired at times of increasing enrollment and the higher quality of those hired at times of declining enrollment. (CMG)