Craighill S. Burks, 56, a principal and administrator in the Fairfax County public school system for more than 20 years before leaving the area in 1967, died Thursday aboard a commercial airliner enroute from Miami to Los Angeles. He apparently had suffered a heart attack.

Mr. Burks joined the Fairfax schools in 1942 as a mathematics and science teacher at Herndon High School. From 1949 through 1951, he was a teacher, science department chairman, and assistant principal at Fairfax High School.

He became the first principal of McLean High School in 1955, supervising, a professional staff of nearly 90, and a student body that was to grow from 1,300 to nearly 2,000 in three years.

Mr. Burks, then in his mid-30s, was one of the youngest principals in the Fairfax County system but had some firm ideas about the job of a school administrator.

He told The Washington Post in 1957 that administrators need to teach periodically because "it keeps you from losing the instructor's point of view."

He was at that time teaching a course in senior mathematics and explained, "It's the best way I know to stay in touch with the everydat tasks of keeping kids out of mischief and motivating them to study."

He also started a policy of putting slow learners in small remedial groups and brighter students in "carefully grouped sections."

"We don't separate smart students from the others entirely and we don't give them more work. But we give them more difficult assignments. It's a matter of depth, not quality," he said.

Mr. Burks characterized himself as a "born optimist," adding, "You've got to be in my job."

He thought that the job of a school administrator was in "providing service to the instructor and student."

He left McLean in 1965, and later served a year as director of secondary education for Fairfax County before moving to New York in 1967 to be headmaster of the Masters School in Dobbs Ferry. He also taught literature courses there.

From 1971 until his death, Mr. Burks was dean of faculty at Ransom-Everglades, a private high school in Miami, where he also taught American studies.

Mr. Burks was born in Berryville, Va., and graduated summa cum laude from Hampden-Sidney College where he earned a bachelor's degree in biology and mathematics.He earned a master's degree in education at George Washington University.

During World War II, he served in the Army in the China-Burma-India theater.

He had been a member of Truro Episcopal Church in Fairfax.

He is survived by his wife, Mary Joyce, a daughter, Rebekah, and a son Timothy C., all of the home in Miami; another daughter, Clary, of Miami, and a brother, Harry H. Jr., of Vienna, Va.