Arlene C. Ackerman, who took the helm of Philadelphia's public schools a little more than a year ago, is pushing for changes that would upend how teachers are paid and assigned to schools. The veteran urban superintendent is battling tradition in the 167,000-student system, but insists that increasing the effectiveness of the city's nearly 10,700 teachers is a central goal of her leadership. Her administration is currently negotiating a new, multiyear contract with the Philadelphia Federation of Teachers. While the changes would be major for Philadelphia, most have been put in place in other large districts, including New York and Chicago. Ms. Ackerman's decision to tackle the touchy issue underscores the prominence of teacher effectiveness on the national school reform agenda.