Any guy who totes a flugelhorn around Washington wearing a neon-green jumpsuit at 10 a.m. is not excessively worried about the wages of nonconformity. In the case of jazz honker Chuck Mangione, occasional flouting of the expected has generally paid off handsomely.

Mangione, known more for the jazz-made-easy sounds of albums like "Feels So Good" than for his earlier work with Esther Satterfied, will be cramping another convention Saturday night when he brings his quartet to Constitution Hall for a concert -- not with Charlie Byrd or Bill Harris -- but with 70 area high school kids. Proceeds from the performance will go to the Special Olympics for mentally retarded children.

This will be the second benefit concert Mangione has staged for Special Olympics, to which he dedicated his recent album, "Fun & Games."

Mangione says the idea for doing the concert, as well as the method, was his. "Usually, when a guest performer plays with a high school band, the music is sent well ahead of time so that the kids can learn it. We're bringing the music with us and rehearsing right along with the kids, so that they'll have nine hours to sink or swim. They'll find out whether they have the kind of endurance it takes to be a musician."

Although more accustomed to working with A-team sessionmen like Steve Gadd and Tony Levin, Mangione enjoys performing with teenagers. "I think it blows their minds, to get this kind of opportunity, it's something they'll definitely remember. And what the kids lack in technique, they more than make up for in enthusiasm.

"People should not expect this performance to be some watered-down high school recital. We'll be doing the whole three-hour set, with full sound and lights, and no opening acts."

Mangione credits his 1978 rise on the pop charts to the Bee Gees. "They had saturated radio (with "Saturday Night Fever") to the point that deejays were looking for something -- anything -- to play that didn't have the Bee Gees sound."

He owes his musical drive to his father, who during the early '50s took him and brother Gap around to the jazz clubs, often inviting restaurant-weary performers like Dizzy Gillespie home for spaghetti, wine and conversation. "I really believe that if we'd expressed interest in becoming doctors, Dad would have taken us around to all the hospitals."

Asked if his more-pop-than-jazz style is evidence that he has sold out, Mangione grins wryly. "Let's see. I've sold out performances at Radio City Music Hall. . . I've sold out performances at the University of California. . ."

THE SHOW -- Chuck Mangione and the Special Olympics Orchestra, Saturday at 8 in Constitution Hall.