A state initiative to evaluate teachers based on a "student impact rating" based on test score growth is facing pushback from school committees and teacher unions ahead of a state implementation deadline.

PaulReville, professor at the Harvard Graduate School of Education, points out that quantifying students' success—especially in certain subject areas—can be difficult.

"Take the typical high school history course," he said. "Do we have any tools available to us to rate that teacher's performance in terms of what students know or how they grow in that subject? Maybe that teacher contributes to the English score—we don't know."

Reville said 'scores' like the student impact rating could be factored in to a larger evaluation of a teacher's skill, but shouldn't be the sole determinating factor.

"I don't think you can take those results into court and make a hard case that this proves this is a good teacher or a bad teacher, but I think it's relevant," he said.

Overall, the renewed focus on evaluation could have a positive impact.

"We've got to be more deliberate and intelligent about evaluating teachers," he said. "We need to be meeting regularly with teachers, particularly novice teachers, to give them feedback on their performance.

To hear more from Paul Reville, tune in to Boston Public Radio above.