New research discovers a bias in teacher-evaluation methods. “School principals—when conducting classroom observations—appear to give some teachers an unfair boost based on the students they’re assigned to teach, rather than judging them solely on their instructional savvy.” The best marks were often given to teachers with high-performing students, while teachers whose students were struggling academically were penalized.

This report, released by the Brown Center on Education Policy, raises concerns about new methods for evaluating teachers. The report suggests “districts try to level the playing field by adjusting teachers’ observation scores based on the demographics of the students they instruct.”

The report also examines other aspects of the teacher-evaluation system. It found that outside observers—rather than principals— “tend to give ratings that are more predictive of teaching quality.”

Although this study does not examine the cause of the bias, it suggests that some teaching skills “may be more difficult with students who are underprepared or not fluent in English.”