The people who run the New York Youth Symphony aren't easy on their young charges. Every Sunday from September through May, the 102 students ages 13 to 21 meet for a four-hour rehearsal, and several times a year the ensemble performs at Carnegie Hall for an audience swelled by the lure of a bargain, namely a $5 ticket. Newly commissioned works are prepared and performed alongside standard repertory, and there seem to be no concessions to youth in the selection of works. There is also no tenure in this orchestra: returning players must audition again every year.
A result of this intensity and discipline is the kind of polished, energetic performance the Youth Symphony gave in the final concert of its 35th season on Sunday afternoon. The concert was conducted by Mischa Santora, the orchestra's 26-year-old music director, and opened with a lush performance of Rimsky-Korsakov's ''Russian Easter'' Overture. There were some tenuous moments the start: in the chant melody in the opening of the work, the winds, heard on their own, produced a slightly mistuned sound, as did the brass. Yet in full ensemble writing, both the winds and brass played beautifully.
The real strength of the orchestra, though, is its string body. In the Rimsky-Korsakov, and later in Strauss's ''Death and Transfiguration,'' the strings played with the warmth and suppleness one expects from a fully professional ensemble. Interpretively, there was also an appealing shapeliness in Mr. Santora's reading of the Rimsky-Korsakov. The Strauss lacked some of the emotional focus necessary to make its mystical transformation seem palpable.
In another Strauss score, the Horn Concerto No. 1, Mr. Santora and his players gave a robust, full-bodied performance, against which David Jolley's solo horn line shone brilliantly. The program also included a premiere, ''The Phoenix,'' by Peter Boyer. Mr. Boyer's work is a Romantic tone painting, untroubled by contemporary harmonic techniques or angular melodies, but rich in the coloristic effects necessary to portray the fiery death and rebirth of the phoenix. The bass and percussion writing was particularly vivid, and the musicians played it with all the power at their command.