Just minutes after leading the newly formed Clef Club Youth Big Band through a rendition of David Raksin's haunting, romantic theme from

Laura

last Saturday, Marc Johnson sat in an adjacent classroom, laughing softly. Through the wall, he could hear the band, composed almost entirely of high-school-age musicians, tackling one of avant-garde composer/bandleader Bobby Zankel's notoriously thorny pieces.

"Some kids would flat-out refuse to play that," said Johnson, an artist mentor at the Clef, pointing a thumb toward the next room. "Bobby's pieces aren't the easiest things to play. But these guys are like, 'You got anything else? Feed me, feed me.' It makes me laugh."

Bemused chuckles seem to be the general reaction to the 18-piece band, a reaction not of mockery but of impressed disbelief. A parade of heads popped in and out of the snug classroom throughout the ensemble's three-hour rehearsal, each emerging a bit stunned that such an impressive sound was coming from such young people.

The band will showcase its talents on Sunday, with its debut performance at the Clef Club under the baton of three conductors: Johnson, Zankel, and Robert Landham.

Perhaps the band's most ardent admirer is its mentor, Clef Club artistic director Lovett Hines. The longtime educator knows a thing or two about recognizing talent - students who have come through his programs famously include modern jazz stars Christian McBride, Joey DeFrancesco, and Jaleel Shaw.

Part of Hines' success with students has been his focus on individuals, which has led him to concentrate the Clef Club's curriculum on small groups rather than big bands. This new ensemble was formed at the urging of the students themselves, with the intention of competing in Jazz at Lincoln Center's annual Essentially Ellington Competition, which features high school bands from across the country. If that goal weren't daunting enough on its own, time was against them; the deadline for this year's competition (which took place the first weekend in May) would leave them only one month to prepare and rehearse.

"I said, 'That's really ambitious,' " Hines recalled with considerable understatement. "Usually, the groups that get involved practice all year round. They start in August. But they still wanted to do it, so I'm overseeing it but the management is the kids themselves."

The band was eliminated in the last round of judging, falling short of admission to the competition by just two points. That disappointment might have put an end to many bands, but the Clef Club group forged ahead.

"Just to be able to play older traditional music is inspiring," said drummer Nazir Ebo. "We tend to play more modern stuff, which is cool, but to go back in history and learn where this stuff came from is amazing. I think every musician should go through that process."

Bassist Rani Barlas agreed, saying, "It's really important to know the roots and where the branches came from." Saxophonist Shyam Natrajan added, "And the Clef Club itself was a part of that, so there's really no better place to do this."

The band, or at least its membership, didn't come away from Essentially Ellington completely empty-handed, however. Pianist Joseph Block, 16, one of the ensemble's founders and leaders, won the Essentially Ellington composition contest with his piece "Volcanic Suite." Block, a junior at Germantown Friends School, conducted the Jazz at Lincoln Center Orchestra in a performance of the piece during the festival in New York, an honor he repeated in Philadelphia last weekend when the Jazz at Lincoln Center Orchestra performed at the Kimmel Center.

"It's important for Jazz at Lincoln Center's Essentially Ellington Competition and Festival to evolve, and it's important for us to have a component that includes new music that is written," Wynton Marsalis, managing and artistic director of Jazz at Lincoln Center, wrote via email. "I know this is something Duke Ellington would have been proud of because throughout his entire career, he fostered new artists and new composers. He embraced new styles, and he loved great musicians. So, I have tremendous respect for what Joseph Block has written. . . . It's difficult to play, but if you can play it, it will be volcanic."

Block said he was incredibly shocked to win the competition, but his thrill was tempered by his disappointment at not having his bandmates beside him. "I got to observe the competition, which was kind of interesting because I thought we were definitely good enough to make it."

That support for the collective seems indicative of Block's leadership qualities. He was in large part responsible for heading the big band, and his mentors all praised his organizational skills, perhaps even more unique than his musical talent for someone his age. He came to the Clef Club at 11, classically trained but with no jazz experience.

"What stands out to me is his maturity, his commitment to the music, and his constant growth," Hines said, before drawing parallels with some of his more famous alumni. "I always say, 'This is the strongest kid I've ever had,' but this is different. He's traveling in the same direction as Christian and Joey, constantly improving and always wanting to try new things."

Clef Club Youth Big Band performs at 4 p.m. Sunday at the Clef Club of Jazz & Performing Arts, 736 S. Broad St. Tickets: $10. Information: www.clefclubofjazz.org.