Thirty years ago, high school big-band jazz was an extracurricular pastime that a few hip students at a handful of schools throughout the nation indulged in after hours. Today it is a prestigious credit activity that has the full support of many school administrations, parents and student bodies. A conservative estimate of the number of high school jazz bands in the United States tops 10,000. Some areas -- California, for example -- have more high school jazz bands, but a strong case can be made that none has better than Northern Virginia.

Take Chantilly High School, whose jazz ensembles have walked off with about 15 high school jazz festival first prizes in the past dozen or so years. Last year, the band surpassed its own record when it was selected high school winner in down beat magazine's Eighth Annual Student Music Awards, a competition for which more than 400 high school bands submitted tapes. Nor do the honors stop there for, as band director Tony Aversano proudly points out, Chantilly's big band graduates have been granted music scholarships right and left, to Boston's Berklee College of Music and other institutions with first-rate jazz programs.

Interested persons have an opportunity to check out Chantilly and other top local bands from 9 a.m. to 6 p.m. Saturday at the Fifth Annual Chantilly High School Invitational Jazz Festival in the school's auditorium. The bands competing include those from schools in the District, Anne Arundel County, and Berkeley Springs, W. Va., as well as the Langley High School Jazz Lab Band. University of the District of Columbia jazz program director and trombonist Calvin Jones, one of the festival judges, will be guest soloist with the Chantilly Band, which will not compete but will wind up the daytime activities with a 5:30 p.m. set. A 7:30 concert will feature the festival's three top bands. The day's and evening's event's are open and free to the public.

WMAL's John Lyon and Ed Walker, who will jointly emcee, credit Aversano for the multiple trophies and scholarships the Chantilly band has won during the 13 years that the Long Island native and Catholic University doctoral candidate has been drilling his students.

"These youngsters today are better technically than the musicians of 40 years ago," says Walker. Lyon adds that "musicians who come through D.C., like Woody Herman, are impressed with the young players that band directors like Tony Aversano are putting out."

"We put a lot of extra hours in besides our regular school time," says Aversano. "And the kids spend a lot of time going to live concerts. The service bands here have some of the best musicians in the whole nation and my students go to their concerts and emulate that fine musicianship.

"I push right from the first day they ever meet me, every day they come into class, I push for them to listen to big-band music," Aversano says. "And when they come into the room I have something playing on the stereo -- Count Basie or Duke Ellington or Buddy Rich, for example. I push for them to build their own record collections and to make tapes of other people's records, to listen to the masters and imitate their styles and then develop their own styles. And I don't limit it to big band -- you can learn to be a good instrumentalist by listening to Frank Sinatra sing."

Aversano is quick to acknowledge that the support system he enjoys, the Chantilly parents group Band Boosters, "takes a lot of the busywork away from me and lets me concentrate on the music." Hard-working festival chairman and father of two Chantilly band graduates Sid Haggard, who was the first to urge Aversano to launch a festival, is a case in point.

"I've quit twice," says Haggard, who has piloted the festival for five seasons. "But I come back every spring."