Educators seeking jobs in the 459 public schools in Northern Virginia and the District of Columbia will have to pass one more test on their way to the principal's office in the future.

George Mason University in Fairfax will operate a new center to assess candidates for principals' jobs in Alexandria, Arlington, Fairfax, Falls Church, Prince William and the District of Columbia, university officials said yesterday. The center is now accepting referrals.

"It's the principal who is the key. The principal provides the climate and direction of a school," said D.C. school Superintendent Floretta McKenzie. The new, more objective process will take some risk out of the selection of principals and their assistants, she said.

At a press conference to announce the formation of the regional center, McKenzie said that she was "very concerned" that teachers, and not principals, have been blamed for what the 1983 "Nation at Risk" report called the public schools' "rising tide of mediocrity."

The study alarmed the public after reporting, among other examples of declining achievement, that the average combined mathematics and verbal Scholastic Aptitude Test scores had plummeted 90 points, on a scale of 800, between 1963 and 1980.

"It's never been more critical to have able leaders in our schools," said Alexandria school Superintendent Robert W. Peebles. "We've made some mistakes over the years in the hiring and firing of principals. We cannot afford to make those today. The stakes are too high," he said.

Previously, principals were hired primarily on the basis of a resume and interview, said Paul W. Hersey, the director of the National Association of Secondary School Principals. Now they will be assessed by a six-member team that will score them over a two-day period in a dozen categories such as judgment, decisiveness and stress tolerance.

The center is the 37th such facility in the country and the second in Virginia; the other is at Virginia Polytechnic Institute and State University in Blacksburg, which opened in July.

The candidates also will be tested in simulated crisis situations and rated on how they handled them, Hersey said.

The new evaluation center, funded by $100,000 from the Virginia General Assembly, nominal fees from the participating Virginia schools and $41,000 from the District of Columbia, will not assess current school administrators.

David H. Lepard, director of the center, called the Metropolitan Area Assessment Center, said veteran administrators might construe this new assessment process as "too threatening," thus only applicants submitted by the school districts will be evaluated.

"We are not interested in revolutionizing education, but gradually improving it," Lepard said.

Many area school systems have already begun to alter their methods of evaluating principals in an effort to raise standards.

"There aren't any permanent appointments any more," said James P. Akin, executive assistant for research, evaluation and planning in Alexandria. "It's perform or perish."

This spring, for the first time, Akin assessed each Alexandria administrator on uniform criteria and rated them on a scale of one to five.

Because eight of the 15 Alexandria principals were rated at three, and one at two, Akin said there would be "some shuffling around." The six administrators who scored above average were given a $1,000 to $2,000 bonus.

Some of the principals who were reached said they welcomed the new system being used by Akin because it means they know exactly what is expected of them, but others said they worried that leadership qualities cannot be measured by the numbers.