The rules change when you dramatize a subject like teen-age suicide on television, because of TV's uncontrollable capacity for promoting imitative behavior in viewers, especially young ones. It would be discouraging if something as tragic as teen-age suicide were to become just another of TV's trendy social topics, this year's answer to divorce or child abuse.

Fortunately, it appears that every impulse behind the CBS movie on teen suicide, "Silence of the Heart," has been honest and responsible. The two-hour film (tonight at 9 on Channel 9) was not only intelligently made, but carefully made. It isn't easy to take, but it shouldn't be. Besides, too much television is.

This thoroughly extraordinary film is an example of a relatively new breed of TV movie that takes newspaper headlines and sociological statistics and gives them human dimension and television's unique immediacy. These films are doing some of the work that documentaries used to do and are devoted to subjects essentially too serious for theatrical films. In short, they are part of an encouraging television revolution.

What's shocking about the current rise in suicides among the young is that there are no easily identifiable causes -- like, say, drugs. The victim depicted in "Silence of the Heart" is, like many in grim newspaper accounts, a seemingly normal member of an affluent, outwardly well-adjusted middle-class family. The film pierces the myth that everyone who commits suicide is "crazy," and tries to analyze the complexities that could drive a bright, likable 16-year-old to the ultimate despair.

"Silence" is a decided improvement over a recent CBS treatment of the same subject on a "Schoolbreak Special" for young people. "Hear Me Cry," which aired earlier this month, was bigger on melodrama than on insight. Tastelessly, CBS bracketed the drama of a young man's suicide with its usual jingly and jivey "Schoolbreak" musical introduction.

The prime-time movie also opens with distractingly jolly music (by Georges Delerue), either to establish the family's life as suburban typical or, perhaps, to lure viewers who might be frightened off by so upsetting a story. But once the film is under way, there is no sweetening or diffidence. At times, it seems stunningly plausible and real.

Skip Lewis (Chad Lowe) is faced with crises that may look trivial to adults but don't to young people: He has scored low on his SATs and now faces the prospect of attending a junior college rather than an Ivy League school, and he is hopelessly smitten with a pretty girl named Andrea who tells him, "I like you, but not in that way, okay?" A conspiracy of circumstances can debilitate someone predisposed to depression, especially someone unequipped to handle deep stress and unable to find adequate psychological help and moral support. That is what happens to the tragic hero of this story.

Most of the film is concerned not with the prelude to the suicide, which occurs in the first hour, but with the reactions to it among the young man's friends and family. The boy's mother, superbly played by Mariette Hartley, wants to believe it was an accident that her son's car crashed through a barricade and over a cliff, but she learns from a girl who had a crush on Skip (Elizabeth Berridge, hauntingly sensitive) that he was despondent and desperate.

The father, played by Howard Hesseman, the only actor in the film underqualified for his role, refuses to accept the truth because he sees it as incriminating; that his son's suicide is his "failure." Skip's best friend, played by an intense and believable Charlie Sheen, is distraught that he failed to see the warning signals and blames himself, almost fatally. But the strongest impression is made by Dana Hill, an extraordinary young actress ("Fallen Angel") as Skip's little sister, at first bitterly uncomprehending, then determined that the truth about her brother should be faced. Her finest moment is a speech to her classmates full of righteous rancor and agonized indignation.

Written by Phil Penningroth and directed by Richard Michaels, "Silence of the Heart" is the kind of TV movie that can shed a lot of light, but there are dangers as well. One of the alarming aspects of suicides among the young is the copycat syndrome that can cause one suicide to lead to others. Obviously when you depict a suicide and attempt to make it understandable, there is the possibility that in some young minds what had been inconceivable begins to make a ghastly kind of sense.

It would appear the producers of the film -- the ambitious Tisch-Avnet team that also does "Call to Glory" on ABC -- have taken the necessary precautions. Perhaps one reaction to the film will be that many parents will ransack their kids' rooms for copies of Sylvia Plath poems; Plath killed herself, and in "Silence" young Skip carries a copy of her work around with him. And some viewers will probably be more alarmed by a nude swimming party in the film than by any of its sober revelations. But for the most part, this would seem a case of an exceptional piece of fiction that could have valuable real-life effects. It's tough viewing, but often the tougher the viewing is, the more rewarding it is as well.