The next D.C. teachers union contract will give principals and teachers greater flexibility to choose longer school days and a longer school year, Chancellor Kaya Henderson told the D.C. Council’s Education Committee on Friday.

The chancellor called additional class time a “key strategy” for boosting achievement and one that has been used at many high-performing public charter schools in the city. Committee Chairman David Catania (I-At Large) agreed that extended school time is necessary to put the school system on an even footing with charters.

The rapid growth of nonunionized charter schools has been an important backdrop to teacher contract negotiations, which have intensified since the previous contract expired in September (although that contract remains in force).

While charters have the flexibility to design their own schedules, traditional public schools are bound by the terms of a contract that says that the year may not exceed 185 instructional days and that a workday must be limited to 7.5 hours.

Washington Teachers’ Union President Nathan Saunders said that although details remain to be worked out regarding longer school days, the union acknowledges “changes that are necessary to make [traditional public schools] competitive in light of the dynamic education environment” in the city.

“But no contract is going to be implemented without the teachers understanding, signing off on and believing” in it, Saunders said.

The last contract was approved in 2010, after a mediator helped end two years of contentious and sometimes bitter talks led by then-Chancellor Michelle A. Rhee and then-WTU President George Parker.

Henderson and Saunders have struck a more genial relationship. They said several months ago that contract negotiations were going smoothly and were expected to wrap up by the end of 2012.

But several weeks after the school system proposed contract language, there are still outstanding issues to be worked out, Saunders said.

Class size is one of them. Education Resource Strategies (ERS), a nonprofit group that advised the school system, recommended raising some class sizes, such as art or music, to concentrate resources in math, reading and other core areas.

“How are larger class sizes better for students?” Saunders asked. “Our members tell me they want smaller class sizes, and, therefore, I advocate for smaller class sizes.”

ERS also recommended ending automatic raises for teachers based on experience and education credits, a move that Saunders said could lead to stagnant salaries and lower pensions for some teachers.

“We need to do some more work on compensation. I’m not happy,” Saunders said.

The school system must find savings somewhere, however. If it meets a goal of having 90 percent of its teaching force rated “effective” or “highly effective” by 2017, it will have to find an estimated $38 million to fund additional merit raises and bonuses for teachers, according to ERS.

Saunders did not offer a timeline for presenting the contract to the council for approval, saying that his priority is to arrive at an agreement that his members will support.

After changes in the Rhee-Parker contract that effectively did away with tenure protections, Saunders said, teachers will “evaluate this contract harshly.”