The union representing D.C. schoolteachers yesterday declared an impasse in contract negotiations with the Board of Education, charging that school labor representatives were "too stubborn to compromise on several key issues."

Washington Teachers Union President Harold Fisher said the union, which represents the city's 4,500 teachers, declared an impasse after 10 months of negotiations. "We have not been able to reach an agreement on salary increases and proposed increases in the length of the school day and the school year," he said.

Fisher said the union is asking the D.C. Public Employee Relations Board to appoint a mediator to work out an agreement between the union and the school system.

The previous contract, signed in 1982, expired in April, but is remaining in effect until a new agreement is reached.

"There is a whole list of problems, but the primary ones are the school board's demand that the school year be increased by 10 days, the school day be increased by a half-hour and salary increases be kept extremely low," Fisher said.

"Our position is that the school year is long enough and there is no justification for increasing the school day," Fisher said.

The union has proposed that teachers receive a 27 percent increase in pay from 1984 to 1986. School officials are countering with a 2 1/2 percent increase next year, an additional 3 percent in 1987 and a one-time 2 percent bonus immediately.

"Their proposed increases are too low. For instance, a 2 percent bonus is, on average, about $610 per teacher. That's an utter insult to the teachers, who have worked hard to bring test scores up and raise the level of student performance in recent years," Fisher said. The average teacher's salary is about $30,000.

"Also, there are no dental benefits and vision-care benefits that we have enjoyed for the past three years in the school officials' proposed contract," Fisher said.

Kenneth Nickoles, director of the school system's labor relations branch, said, "The bottom line is that we are at impasse. There are a number of issues that separate the parties, but school officials have no comment on the issues at this time."

Three years ago, union and school officials failed to reach agreement on some of the same issues over which they are currently at odds and an impasse was declared by the union.

The last salary increase for teachers was in 1983 when they received an 8 percent raise. Before that there were increases of 7 percent in 1982 and 6 percent in 1981, Fisher said.

Superintendent Floretta McKenzie "has been adamant that test scores have gone up in this system, but she has not shown that she supports adequate increases in pay for the teachers who made these improvements possible," Fisher said.

In addition, he said, "There is an inevitable teacher shortage coming up and school districts across the country are competing for the best teachers. D.C. will not be able to attract the best teachers unless the salaries are competitive."