Structurally unsound but emotionally authentic, "Baby, It's You" is bright young writer-director John Sayles' first major studio project. Financed on a modest budget (roughly $3 million) that nevertheless dwarfs the combined costs of two other Sayles films, "Lianna" and "The Return of the Secaucus Seven," "Baby, It's You" reveals that it does Sayles no discernible harm to acquire a more professional filmmaking style.

In particular, the acquisition of a capable cinematographer, the German Michael Ballhaus, making a crisply realistic American debut after years of lighting the glossier Fassbinder spectacles, seems to free Sayles for the first time to concentrate on the immediacy and authenticity of settings and performances.

Opening today at the K-B Cerberus, "Baby" is the chronicle of a high school infatuation that entrances a fundamentally mismatched pair: Rosanna Arquette as an upper middle-class Jewish girl named Jill Rosen, who is headed for college and more or less certain respectability, and Vincent Spano as a lower middle-class Italian American dreamboat, who calls himself Sheik and cultivates an ostentatiously dapper, devil-may-care, insubordinate image that barely conceals a woeful lack of smarts or future prospects.

Sheik, a vaguely notorious transfer from a parochial school, puts the rush on Jill, a top student and leading senior thespian, when he materializes on the campus of a Trenton public high school in 1966.

Stimulated by what is obviously a pretty basic as well as dynamic challenge--Sheik's magnetic gaze persists in asking the implicit question, "How about it, baby?"--Jill remains emotionally spellbound despite a number of alarming indications of character defects in Sheik.

It's the sort of attachment that could only delude the sweethearts themselves within the oddly promiscuous, leveling social atmosphere of a large American public school, where the social and intellectual gulf that separates Jill and Sheik can be obscured temporarily by generational solidarity and sheer sexual attraction, curiosity and ignorance. Removed from that atmosphere, the infatuation is virtually impossible to sustain; differences in class and taste begin to matter.

The movie derives a considerable amount of revelatory poignance and originality from a close scrutiny of this usually neglected or unacknowledged social tension.

Jill and Sheik struggle to revive an unconsummated and already nostalgic romance when the next academic year takes them in different directions, emotionally as well as geographically. While Sheik pursues a vain show biz dream in Miami, Jill enters her freshman year at Sarah Lawrence.

Now a jittery, overwhelmed newcomer to a more privileged, competitive and potentially snobbish educational environment, Jill is lonely enough to feel pangs about Sheik, but at the same time she begins outgrowing and betraying her attachment to him, adapting to a setting that decisively excludes him.

Not that Sheik ever would do as a mate for Jill, but she's naturally reluctant to admit the sort of fact that would be obvious to, say, her parents. A sensitive and skillful young actress, Arquette keeps a self-aware note of shamefulness underneath Jill's coed jocularity when she violates the feelings Sheik actually evoked in her and eventually turns him into a legend suitable for communal facetiousness; joining a dorm pot circle as the freshman year matures, Jill contributes some slighting anecdotes about the heavy-duty Jersey greaser-loser she was once crazy enough to date.

Although Sayles has the sole screenwriting credit, the core authenticity of this absorbing, melancholy teen-age love story would appear to derive from the experience of producer Amy Robinson, who gets a story credit and bears an uncanny resemblance to Jill Rosen when one consults her biography. In fact, the story unfolds so preponderantly from the point of view of the heroine that Sheik's side of things is certainly obscured well before Jill is prepared to admit that he's a loser.

There's an element of deception in this imbalance that may be dramatically expedient, but it's never quite kosher and leaves a number of nagging social questions unanswered.

Being circumspect about Sheik may prevent the magnitude of his dumbness from dawning earlier than the filmmakers want. Nevertheless, he behaves dubiously enough as a high school Big Operator to invite a measure of incredulity about Jill's devotion to him.

There's no denying the personal magnetism of Vincent Spano, who is blessed with a wonderful, sharp-beaked romantic profile and made himself nobly ingratiating only a few weeks back as the Berber youth who befriends Kelly Reno in "The Black Stallion Returns."

But Spano's appeal and Sheik's are not interchangeable. As conceived and written, Sheik always seems a charity case.

The story also seems to be on the verge of ending for about half an hour before it really does. It was probably this defect that caused Sayles so much reported conflict with studio executives over the script.

Still, it's worth sticking out the bumpy and draggy stretches for the scenes that achieve a funny or excruciating fidelity to life, and the scenes are at least reinforced by an evocative sound track of rock oldies.

When Sayles' ear for dialogue and his young performers' talents are effectively coordinated, "Baby, It's You" has a distinctive bittersweet tang. There's a dead-accurate comic plaintiveness in Rosanna Arquette's voice, for example, when Jill reveals her freshman insecurity in a beautifully concise line: "It's different here; I don't know how you're supposed to act." BABY, IT'S YOU

Director, John Sayles; screenplay by John Sayles, based on a story by Amy Robinson; director of photography, Michael Ballhaus; edited by Sonya Polonsky; costumes designed by Franne Lee; associate producer, Robert F. Colesberry; produced by Griffin Dunne and Amy Robinson for Double Play Productions. Released by Paramount Pictures. Rated R. THE CAST Jill . . . . Rosanna Arquette Sheik . . . . Vincent Spano Mrs. Rosen . . . . Joanna Merlin Dr. Rosen . . . . Jack Davidson Mr. Capadilupo . . . . Nick Ferrari Mrs. Capadilupo . . . . Dolores Messina