THE TEACHERS' contract now under negotiation in the D.C. schools is extremely important. The city must play catch-up to the significantly higher teacher salaries that prevail in the suburban school districts. But in return the city system should be able to exact concessions from the teachers' union, so that if salaries are equalized, duties will be too.

Though enrollment is not changing much, the city finds itself in the position of needing to hire 400 to 500 teachers in each of the next several years. That is far more than in suburban school districts. A major reason is that many older instructors are due to retire. A second factor is the continuing effort to reduce class sizes.

The climate is competitive. The trend around the nation is toward significantly higher pay. In Rochester, N.Y., for example, the starting salary for teachers will soon increase to $29,000. The Montgomery County schools had a starting salary of $22,000 this year, due to rise to $25,661 by 1989-90. The beginning salary in the District this year was, by contrast, $19,116. The salary gap is wider higher up the pay scale. That is true even though in some respects the city is a harder place in which to teach. City teachers also receive far smaller stipends for running extracurricular activities.

But in order to earn more competitive salaries, the city's schoolteachers ought to be willing to meet more demands. D.C. teachers, for example, are required to be at school for seven hours a day. That's 30 minutes less than in every neighboring school system. (City students are at school six hours, suburban students generally six-and-a-half.) District teachers are required to spend 275 minutes per day in actual instruction. That compares with about 330 minutes a day in suburban schools and as many as 390 minutes in other large cities. A teacher evaluation system that matches performance with higher pay is also needed. The old D.C. teachers' contract, which expired on Oct. 1, contained a provision barring principals and other supervisors from asking teachers to show them detailed lesson plans in most circumstances. That can be scrapped.

A good contract would be a trade. More would be both given to and asked of D.C teachers. The result would be the better school system that presumably is everyone's primary goal.