D.C. Schools Chancellor Michelle Rhee will fire a large number of teachers and school staff by next week based on ineffective performance, said a top aide -- a move certain to become a factor in the tight mayoral race.

"It's going to be a sizable number," Deputy Chancellor Kaya Henderson said at a briefing Thursday about details of D.C. Public Schools' evaluation tool. The tool, called Impact, was used for the first time in 2009-2010 to determine employees' effectiveness, based mostly on five observations as well as measures of students' academic progress. Teachers evaluated as "highly effective" will be eligible for performance bonuses in 2010-11.

Rhee declined to comment on the numbers, expected as early as Friday.

Even if the number of fired employees -- from teachers to librarians to custodians -- is as small as 50, the move would be almost unprecedented nationwide. For decades, school employees' contracts have made it almost impossible for districts to fire them based solely on substandard performance.

A round of nearly 300 DCPS firings in the fall, based on a budget shortfall, led to fierce protest from school and union faithful. Mayoral challenger Vince Gray took up the cause of the offended, saying the firings were based on a phony premise. Mayor Adrian Fenty has stood firmly behind Rhee's no-apologies approach to ousting low-performing teachers from the system, regardless of tenure.

A city official speaking on background said about 20 termination letters have gone out thus far, with more expected in the week ahead.

Henderson refused to provide specific numbers, saying final calculations are not complete. However, she did not balk when asked if it could be as high as "a couple of hundred."

"There were about 6,600 employees evaluated," she said, implying that even if only 2 percent of them ranked "ineffective," that would total about 130.

Washington Teachers' Union President George Parker said grievances would "definitely" be filed for each fired teacher, based on what many teachers perceived as a botched process.

"The [Impact evaluation] is flawed, and we requested that DCPS not terminate teachers or inflict adverse actions in its first year," Parker said. "The document should've been piloted first."

A union survey in early 2010 revealed significant complaints about Impact, but education think tanks have applauded it for its comprehensive approach to measuring teacher performance.

The results of the evaluations -- compiled in a new online database instead of filed away in school cabinets around the city -- will be used to tailor professional development much more specifically than in past years.

"The possibilities are endless now that we have this information," Henderson said.