University of Chicago News, via a kind Beth Dorhn email:

A bonus payment to teachers can improve student academic performance — but only when it is given upfront, on the condition that part of the money must be returned if student performance fails to improve, research at the University of Chicago shows.
The study showed that students gained as much as a 10 percentile increase in their scores compared to students with similar backgrounds — if their teacher received a bonus at the beginning of the year, with conditions attached. There was no gain for students when teachers were offered the bonus at the end of the school year, the research found.
“This is the first experimental study to demonstrate that teacher merit pay can have a significant impact on student performance in the U.S.,” said UChicago economist John List, an author of the study.
The study, “Enhancing the Efficacy of Teacher Incentives through Loss Aversion: A Field Experiment,” published by the National Bureau of Economics Research, reflects the findings of other studies in psychology and behavioral economics.