A free public concert on Saturday by the Northern Virginia Regional High School Orchestra will feature 109 top student musicians in an annual community event designed to showcase the quality of the state's educational music programs, according to Amy Lavo, chairwoman of the event.

The 8 p.m. performance in Chantilly Secondary School will be the culmination of years of study and months of work for talented school musicans, each of whom competed for a seat in this year's youth orchestra. Outstanding players from the original group are then eligible to perform with Virginia's All-State Orchestra in April.

More than 200 tenth and eleventh graders from Alexandria, Arlington, Fredricksburg and Manassas cities and Fairfax and Price William counties auditioned during September. The winning 109 students will have been practicing privately almost two months since then for two days of rehearsals and one evening's performance this Saturday.

Julie Gigante, 16, a junior at Alexandria's Fort Hunt High School, said she began to take music instruction at age 8 and has now devoted more than half of her life to the serious study of piano and violin. Julie will serve as concert-mistree during rehearsals and at the performance.

As concertmistress, Julie will function as a musical aide to conductor Dingwall Fleary.

"The concertmistress has to get the strings organized and see that they bow together," she said. "If anyone in the string section has a problem fingering or wants to know how to play a certain passage during rehearsals, they see me.Sometimes they want to know to bowing - whether to bow up or bow down. You wouldn't believe how complicated that is. That's how you tell a mediocre orchestra, if they don't bow together.

In order to perform with the Northern Virginia Regional High School Orchestra this year, Julie had to go through a blind autition (the judges couldn't see the students) and pass three musical evaluations.

"You had to perform a required etude, do a solo of your own (she chose the first movement of the Bruch Concerto) and play some scales," she said.

Julie first performed as a concertmistress "in the sixth grade," she said. She now plays first violin in nearly a half-dozen community groups and has won innumerable awards. She said she was particularly proud "being named best instrumentalist at Fort Hunt High School in 1976 and winning the Arlington Symphony's youth scholarship contest, senior division."

Julie studies locally with a husband and wife teaching team in Arlington, Maryen Herrett (piano) and Carlton Herrett (violin). She also spent last summer studying for college credit with the Bostom Symphony's young-artists instrumental program at Tanglewood Music Festival in Massachusetts.

Like Julie, most of the young musicians in this year's regional orchestra started their music study at the elementary school level, according to Amy Lavo.