In the world of Scott Joplin and his peers, the New England Conservatory Ragtime Ensemble holds a place comparable to that once held by Rubinstein in the music of Chopin or that still held by the Juilliard Quartet in the quartets of Bartok, Saturday night in the Kennedy Center Concert Hall, these 18 students from Boston's leading music school, directed by Gunther Schuller, demonstrated once again how they have achieved such eminence in their specialized field.

The secret of their success is twofold: depth of repertoire and polish of performance. Joplin's justly popular "The Entertainer" opened the program and was played again as an encore - quite properly, for if one piece of music is the key to the Joplin revival, this one is it.

But everyone plays "The Entertainer" and "Maple Leaf Rag," those who got to hear the New England Conservatory Ensemble receive a handsome bonus in such relative rarities as Artie Mattews' amazing "Pastime No.4," which could be mistaken for music of Stravinsky; Tom Turpin's "Harlem Rag," which is the first published ragtime composition, dating from 1897; the "Ragtime Nightgale" of Joseph Lamb, which is pratically a small tone-poem, and the "Cataract Rag" of Rob Hampton, which includes piano passages that might have come from Beethoven 's "Emperor" Concerto.

The concerts are also given deoth by Schuller's running commentary including the touching story of how Joplin finished "Heliotrope Bouquet" for a dying colleague and the story of James Europe, who died at the beginning of a career that might have meant a rel fusion of African and European musical idioms.

As for the performance, it was expert and spirited; the winds sometimes outbalanced the strings, but that seems appropriate for the music.