In a close vote, teachers at the Green Dot charter school chain endorsed a merit pay plan tied to test scores.

Although test score-based evaluation is highly unstable, the teachers decided to go along in hopes of qualifying for a bonus.

A teacher rated effective one year may be rated ineffective the next year, because there are so many factors beyond the teacher’s control that affect student scores.

The National Council on Teacher Quality thought this was a good move. So did Green Dot CEO, Marco Petruzzi, who previously worked as a management consultant at McKinsey and Bain Capital.

Some teachers were not happy with the decision. Some were suspended or fired for fighting it. Students joined with teachers to protest, and the administration said the whole thing was blown out of proportion.

Scholars have warned that this method of evaluating teachers encourages teaching to the test, narrowing the curriculum, and other negative behaviors. Teachers who teach special education or English language learners will see the smallest gains. If these groups are underrepresented at Green Dot, as they are in many charter schools, that won’t be a problem.