D.C. teachers finished voting yesterday on their proposed three-year contract, but their ballots, sent in by mail, will not be counted until Saturday afternoon.

Washington Teachers Union president William H. Simons said about 50 percent of the ballots were already in. The ballots must be postmarked no later than Wednesday. "I'm quite positive the agreement will be ratifed," Simons said. The ballot counting will take place at the union's downtown offices.

The proposed contract gives teachers an across-the-board 21 percent salary raise over its three-year duration, more than any other school employes will receive but slightly less than the city's police and firefighters won.

Many teachers have complained, however, that the contract will require them to work an extra half-hour a day and an extra four school days each year.

The teachers also have criticized a contract provision that would require them to earn a better-than-satisfactory rating in their sixth year on the job in order to receive their salary increase of about $700 for that year. Currently, teachers need only receive a satisfactory rating at that point in order to receive a raise.

At the same time the contract gives teachers greater authority to protest the job performance ratings they receive from principals. Currently teachers can protest only unsatisfactory ratings.

The contract gives the union the right to collect 50 percent of its annual dues from teachers who are not union members. About 4,200 of the system's 5,500 teachers are union members.

If the contract is not ratified, negotiators for both the union and the D.C. school board will go back to the bargaining table. If a new agreement cannot be reached, the contract will go to binding arbitration.