With chins in and eyes straight ahead, nine teams of public high school cadets marched in snappy precision and jazzy synchronization yesterday, competing for trophies in the District's annual junior Reserve Officer Training Corps drill competition.

Top prizes went to Phelps Vocational School and Ballou and Coolidge senior high schools. About 1,000 cadets participated in the events at Eastern High School. Phelps took first place in the battalion competition, Ballou won two categories of drill team competition, and Coolidge took the platoon competition. Judges rated them in several categories, with special emphasis on the teams' innovative movements and routines, military bearing and appearance.

During the Vietnam War era, the numbers of ROTC cadets declined dramatically. Today, however, as is the case at many colleges across the country, the number of cadets in the public schools' ROTC program has increased, said Col. Raymond Smith, coordinator of the District's ROTC program, which he said has grown from 600 cadets in 1974. "We're experiencing a trend. In 1981, there were less than 1,000 cadets. Last year, there were 1,200 and this year there are 1,420."

Smith said that the tight job market is a factor in the increased number of cadets. "Before, very few cadets joined the armed services, now many do," Smith said. "Economics has a lot to do with it." So does good public relations, according to another Army official. "Kids are walking down the school hallways chanting: Army, Navy, Air Force, Marines, just like in the commercials," he said.

The Washington High School Cadet Corps, established about a century ago, has held annual cadet competitions like the one yesterday on sports fields for nearly 100 years and is a city institution. For many years the event provided a splendid, festive occasion complete with family picnics and VIP attendance, local ROTC officials said.

"It was bigger than football," said Col. Smith. "People used to walk from all areas of the city to be there. It was a holiday, really. But, over the years, the luster of being a soldier wore off."

At the competition yesterday, Army Cadet Sgt. Thad Jarmon, a sophomore at Coolidge, said he joined the corps because he needed discipline. "I was real wild, just like a lot of my friends, but I knew that being like that wouldn't get me too far in life."

He was one of about two dozen cadets competing for trophies in the individual drill category. Like a toy soldier, the 15-year-old responded to the commands of a marine drill sergeant with near perfection.

"Atten-shun! Riiiiiight face! Leeeeeft face! A-bout face!" the stern-looking drill sergeant barked repeatedly until all but three cadets were eliminated.

Jarmon won second place. "I felt a little nervous at first, but then I just started to go with the flow. I feel real proud of myself."