In his first formal appearance before Prince William County's teachers, Superintendent Edward Kelly sounded a clarion call yesterday for the vocation of teaching, calling it "the noblest of all human endeavors."

Kelly, who addressed the 2,255 teachers, principals and librarians in this 38,800-student system in three sessions, gave little indication of his specific priorities or plans.

Instead, in a speech tinged with the rhythms of the pulpit, he urged staff members to take pride in their work.

"You are laying the groundwork for our future," he said. "Teaching is not mere employment. It is more than a career. Teaching is a calling."

Teachers at the session for middle-school personnel gave Kelly a kelly green T-shirt inscribed with a quotation -- "Nobody Knows Kids Better Than Teachers" -- for which Kelly takes credit. The superintendent received a standing ovation.

Kelly, 45, former school superintendent in Little Rock, Ark., assumed his $82,000-a-year job in July.

His predecessor, Richard W. Johnson, was fired by the county school board after seven stormy years as superintendent.

"Kelly was certainly very positive and held the attention of the audience," Brenda Weaver, a librarian at Bel Air School, said of the afternoon assembly at Woodbridge High School.

"The teachers are very anxious to know him better," said Maryann Bannwart, a third grade teacher at Bennett Elementary School.

"We've been through a very bad experience," she added, alluding to a dispute between teachers and Johnson.

Also on the program at yesterday's assemblies was Stephen Glenn, a California specialist in staff development, who spoke on "Techniques for Teachers to Build Capable People."

At its meeting last night the Prince William School Board authorized its staff to hire up to 15 additional teachers and six additional aides if enrollments exceed the projected number of 38,974.

In previous years, additional people have been hired to meet enrollment demand; 32 were hired in 1986-87 and 38 in 1985-86.

The staff also presented a proposed AIDS education program for seventh grade science classes; eighth, ninth and 10th grade health and physical education classes and Biology I.

The board will consider the curriculum, which includes videotape and instructional materials, at its next meeting, Sept. 16.