EIGHTY-FIVE PERCENT of D.C. teachers were judged to be effective or highly effective in 2010-11 under the system used to evaluate school personnel. But less than half of their students were proficient in math or reading. Clearly, something was out of whack. That’s why Schools Chancellor Kaya Henderson is on solid ground in raising the standard for what it means to be effective.

District school officials last week announced the “most extensive overhaul” of the IMPACT system that was ushered in three years ago by former chancellor Michelle A. Rhee. The aim of IMPACT was to replace the subjective and haphazard way teachers had been evaluated with a system that set clear expectations, reliably measured performance and provided support, but that also had consequences. There’s been no question of its effect: Almost 400 teachers judged not up to par have been dismissed, standout teachers have been rewarded with some of the highest salaries in the country, and other school systems, as The Post’s Emma Brown reported, have followed the District’s lead in linking teacher pay and job security to student achievement on standardized tests.

So dramatic — and controversial — was the District’s approach (the mass firing of teachers was surely a factor in former mayor Adrian M. Fenty’s defeat in 2010) that some predicted that IMPACT would eventually be watered down, if not abandoned. That the school system announced the dismissal of 98 teachers at about the same time it was unveiling the changes helps put that notion to rest; Mayor Vincent C. Gray (D) has shown no sign of retreating from school reform. Moreover, the changes that will go into effect at the start of the 2012-2013 school year, taken as a whole, make for an even more rigorous system.

Particularly noteworthy are the revisions to the rating system that make it harder to be judged effective and create a new “developing” category in which teachers are given three years to improve or face termination. If the new standard had been applied to the 2010-11 data, the percentage of highly effective and effective teachers would have decreased from 85 percent to 51 percent, a figure much more in line with the competency of students.

It’s a tad disappointing to see that standardized test scores will carry less weight in a teacher’s rating (35 percent instead of 50 percent): Student achievement is the true measure of a teacher, and the value-added measurement, which takes into account factors such as poverty, is still judged to be the most accurate. But school officials make a strong argument that the 50 percent weight was having the undesired effect of unsettling, and possibly driving away, its most effective teachers. That the system will employ other assessments of student achievement and will still insist on multiple teacher observations is further reassurance of the commitment to rigor.