Nearly 100 D.C. Public Schools teachers were fired this week for poor performance after they received low ratings on their teacher evaluations, some based heavily on students' test scores.

The 98 teachers represent about half of the teachers fired last year, while the number of top-rated teachers grew from 663 to 998.

The decisions were made based on controversial evaluation tool Impact, developed by Chancellor Kaya Henderson when she was the deputy to former Chancellor Michelle Rhee. Under Impact, teachers whose classrooms are tested on the D.C. Comprehensive Assessment exams receive 50 percent of their ratings based on how well their students progress on the standardized tests.

The District's school system is a rarity in the nation, one of the first to link teachers' evaluations to job security and one of the few to follow through on terminations after those ratings.

About 400 teachers have been fired for poor performance on the evaluations since 2009, The Washington Post reported Thursday.

"We owe it to our teachers to provide them with more and better curricular resources and professional development to ensure their success -- and we're doing that," Henderson said in a statement she gave to The Post. "But we owe it to our students and families to continue to move out the professionals who are not up to this incredibly difficult task."

Teachers can receive ratings from "ineffective" and "minimally effective" to "effective" and "highly effective" on the evaluation tool. Of the fired teachers, 39 were rated "ineffective," while 59 were found to be "minimally effective" for a second-consecutive year.

Those rated "highly effective" are eligible of bonuses up to $25,000. A spokeswoman for Henderson said the bonuses would cost about $8 million this year, to be funded by private donors.