Chancellor Kaya Henderson, under pressure from the Washington Teachers’ Union, has loosened rules governing the IMPACT evaluation system that could allow some veteran educators who receive two consecutive poor appraisals to keep their jobs.

It’s not clear how many teachers will be effected by the change. Separation notices based on 2011 IMPACT scores are expected to go out as early as Friday.

But it is a potentially significant change in the two-year-old assessment system developed under former chancellor Michelle A. Rhee to raise the quality of teaching in the District. It is also likely to raise new questions about the union’s political influence and the resolve of Henderson and Mayor Vincent C. Gray (D) to follow through completely on Rhee’s signature initiative.

In 2010, IMPACT’s first year in operation, 126 teachers were fired for poor performance and another approximately 500 were judged “minimally effective.” Under the original rules, they had another year to improve or were subject to dismissal. The adjustment began last month when DCPS human capital chief and IMPACT architect Jason Kamras told principals that if they had second-year teachers with promise who seemed to be headed for a second consecutive poor evaluation, they could apply for exceptions.

Kamras said the change was justified by a wide body of research showing that new teachers can improve dramatically in their first few years of practice. But the exception drew a vigorous protest from WTU president Nathan Saunders, who said it discriminated against older, more experienced teachers who also had been working under IMPACT for the last two years. A series of conversations with Henderson followed.

Saunders also took his case to Gray, who received heavy union support in his 2010 mayoral campaign. Saunders said Gray did not promise to intervene with Henderson and Kamras, but added:

“He always defers to Kaya, but when I remind him of how he got to be mayor and where he was on issues like this as [D.C. Council] chairman he’s always thoughtful.”

Henderson decided to broaden the exception, and Kamras informed Saunders of the change last weekend. Saunders told union members in a robocall earlier this week.

Still waiting for comment from Henderson and Kamras.