What can you say about a 17-year-old girl pianist who makes Mendelssohn sound like Mozart?

Cecile Licad this weekend completed her second season at the Marlboro Music Festival, that astonishing conflagration of talent, with the Mendelssohn Sextet in D major.

Looking about 12 years old, she electrified the audience with the force of her playing, the quicksilver clarity of her fingerwork, the intensity and drive that not only made the often-soggy Mendelssohn crackle with energy, but charged up the rest of her group: four other young women and a veteran Julius Levine on double bass.

Diane Monroe, the violinist, sensitively connected with Licad, who sometimes seemed impatient with the delicate pauses, waiting tense as a whippet for her next intrance. The cheering, stamping audience response exceeded even that for the difficult Tchaikovsky Sextet in D-minor, a complex work completed in the composer's last years.

First violin Peter Zasofsky and especially violinist Michael Tree, who is a member of the Guarneri Quartet, emphasized the passion and fire contained in this work, so Italian and so Russian, but never at the expense of logic.

For those who had heard the rehearsals - Marlboro being at heart a workshop, where playing is more important than performing - the final version of this piece lacked the abandon of earlier practice sessions. On the other hand, the musicians seemed better balanced in performance and they had conquered the temptation to accelerate in the fast passages.

The Bert String Quartet rounded out a rigorous and exciting evening. Played by a mostly young group who took naturally to its romantic melancholy, the piece was dominated by first violinist Hiroko Yajima, another woman musician of great drive and sure command.

Second violin Mitchell Stern, given a larger part than one normally hears in a quartet, matched his colleague with wonderfully sonorous responses. One does not call the 70 young musicians here students. Merely to enter Marlboro musicians must have mastered their technique, and be able to play everything in their instrument's repertoire.

The two-month Marlboro season ended Sunday with the traditional presentation of Beethoven's triumphant Choral Fantasy.