Like an early drawing for a great future fresco

American Ballet Theater (ABT) began an unusually long season (to end July 16) at the Metropolitan Opera with one of the most highly anticipated events of the season: a revival of George Balanchine's ''Symphonie Concertante,'' created in 1945.

Considered a ''lost'' ballet, because it was never performed after Balanchine let it lapse from the New York City Ballet repertory in 1952, ''Symphonie Concertante'' is now restored, thanks to a happy happenstance. A ballet master of ABT stumbled on an old score of the ballet in the archives of the Dance Notation Bureau, and from this notation system a reconstruction was possible.

This retrieval of what was commonly thought to be lost comes at a poignantly right time. Balanchine, who passed on last month, left a massive body of extant ballets, but also a large number of works whose viability is tenuous unless ballet masters see to it that they're performed. As with all the great artists of all the centuries, we want to see more than Balanchine's masterpieces. It's necessary to know the complete artist - and that means his middling range, too.

''Symphonie Concertante'' falls in that range. Its chief interest - and the reason for the excitement it has provoked among the public - is that it's by Balanchine. Furthermore, it's Balanchine working with Mozart - the composer to whom he's most often compared, but, ironically, one he used infrequently. The ballet is a vast ensemble piece, set off by a cavalier and two ballerinas, one taking the violin-solo the other the viola-solo passages of Mozart's score in E-flat major, K. 342 .

In its architectural use of the ensemble as a way of achieving grand scale, ''Symphonie Concertante'' is a model for the resounding masterpiece that came two years later, ''Symphony in C'' to Bizet. Thus, the Mozart ballet is like the preliminary drawings of a great fresco. Originally made for students, the ballet is also a marvelous example of how a choreographer can make an ensemble of limited capability look interesting, simply through the use of pattern and the dramatic effect of watching a lot of people doing the same steps.

Balanchine's means are transparent in this ballet, which makes it overly academic yet superbly educational. If I were a choreographer, I'd hotfoot it to the Met with a notebook in hand.

There are also moments of genius, when the emotional resonance of the steps makes one drop one's pencil in wonderment. Growing out of the interplay between the two ballerinas is a feeling of tender friendship and respect for each other's domain. The genius aspect is in the way Balanchine carries those feelings into the ballet's general fabric.

Every so often, for example, the ensemble joins hands for a second as an indirect reference to the soloists - and, as always, the indirect is the most powerful. Then there is a miraculous trio for the ballerinas and cavalier - miraculous because the man, rather than bringing out the duality of the women, acts as a mediator between them, drawing them even closer together as he partners first one, then the other. He becomes an invisible, or secret, binding force.

These great moments are, of course, moments. ''Symphonie Concertante'' is primarily an artistic document of a choreographer who merits all the documentation he can get. Robbins's work

Social documentation is the prime interest of Jerome Robbins's ''N.Y. Export: Op. Jazz,'' which ABT also revives this season. This jazz ballet is a definitive record of one aspect of the 1950s, the cool, aggressive behavior of alienated teen-agers. Some may challenge the subject's triviality and wonder why it's of interest to an artist of Robbins's stature. Be that as it may, Robbins has truly captured the essence of a mood and a time.