Their Little Gymnast Has AIDS : AT RISK <i> by Alice Hoffman (G. P. Putnam’s Sons: $17.95; 210 pp.) </i>

Polly and Ivan Farrell live in a gracious Massachusetts seaside town, so gracious that Harvard graduates accept low wages to teach in the local school, making it one of the 10 best in the state.

Polly, a photographer, is 36 but looks like a graduate student. Ivan, an astronomer, is tall and amiably disheveled. They have a comfortable marriage, good sex and enough quirkiness to forestall total relaxation.

They inhabit an old white house with black shutters and a big porch where, on summer evenings, they watch the fireflies and their two children and think how perfect things are. Amanda, 11, is a passionate and promising gymnast, made, Polly reflects, “not of flesh but of points of light.” Charley is a small, absent-minded scientist who mucks about with snails and bugs.

So begins Alice Hoffman’s “At Risk,” dosed to faint excess with the comfort and comfortable jokiness of an upper-middle-class situation comedy. It is about to turn, if there is such an expression, into a situation tragedy. One summer day, Amanda develops a heavy sweat, fever and swollen joints. A transfusion of contaminated blood has given her AIDS.

Situation comedy, situation tragedy--it may seem a harsh judgment. Hoffman, author of “Illumination Night,” has a style that can be delicately bewitching and capable of a streak of wildness. Sometimes the streak and the delicacy are evident in “At Risk,” but they have a lot to contend with.

A will-o’-the-wisp talent has hitched itself to the ponderousness of a problem novel. Each character and situation feels calculated to illustrate still one more aspect of the individual and social awfulness of AIDS, to make us think “This can happen to us” and to touch us. It touches, all right, but we have been turned into a fixed target, maneuvered into range. With frequent grace and with an impressive ability to work out the variations and consequences of hurt and panic, Hoffman has produced a high-minded tear-jerker.

Sometimes it seems as if “At Risk” is staged as much as written. The suburban felicity into which the disaster will erupt is carefully mounted. There are the Farrells’ talent, good fortune and good humor. There is a note of amiable clownishness. Polly is able to grow nothing but zucchini in her garden; she sneaks it into the dinner meat loaf and serves it up as onion rings. When Ivan is summoned to capture a wasp, he puts a colander over his head. Bad things can happen to the successful but not, surely, to the winsome.

But there is a dark spot or two in the lighting, a hollow bass note in the incidental music. Mention is made of a tuberculosis epidemic that decimated the town population early in the century. Polly’s photographic work has included studies of the dying; at present, she is working on a book about a medium who communicates with the dead. The weather is terribly hot until it breaks, in a lighting shift, to coincide with the discovery of Amanda’s illness.

It is a kind of stacked deck. On the other hand, Hoffman plays the cards out with skillful tension and a commanding emotional imagination.

We know the bad news before the family does; we see the family doctor (and friend) getting a phone call from the laboratory and bracing himself for what is to come. We see Ivan summoned to his office. We don’t see him being told--Hoffman’s skillful delicacy includes omitting obvious climaxes--but we see him parking the car on the way home and howling.

There is painful suspense as we wait to read how Polly and Charley will react and above all, of course, how Amanda will. The victim thinks first of all about her gymnastics practice. Her insistence on keeping on with it as long as she can provides a theme that is both moving and absolutely authentic.

Amanda is the touchstone of fragile reality in a nightmare where all bearings are lost. As she gets sicker, she insists on asserting what others forget; that she is still alive. She learns how to make chocolate mousse from an adult friend who, precisely because she is less involved in the family, is the only one Amanda can talk to about death.

Just before the end, she insists on having her braces removed. Looking at herself in the mirror, she smiles, Hoffman writes, “because now she knows. She would have been beautiful.”

That may be the most heart-rending moment in the book. But we also get a precise cataloguing of the pain and disruption that take place in the Farrell family and in the community outside.

Hoffman portrays convincingly the distances that, out of sheer self-preservation, set in among the close-knit Farrells. Polly and Ivan pull apart, barely communicating. Their channels of openness have to be closed down, or their pain will burn them both out. In a disaster of this kind, love can be the enemy of intimacy.

Just as Amanda has sought a confidante outside the family, Ivan pours out his own feelings to an AIDS victim who is operating a hot-line network; and Polly turns to their doctor for comfort and an inkling of something more. Only Charley has no recourse. Nobody washes his clothes--a neat image of abandonment--and he retreats into a kind of hermetic stoicism, punctuated by nightmares.

Friends and acquaintances turn variously and unpredictably supportive or aloof. The school is under pressure; a number of parents withdraw their children. There is a fuss about toilets and about the gym’s practice bars.

The principal and Amanda’s coach demonstrate valor and compassion. A woman who is a close friend of Polly’s forbids her son to associate with Charley, his best friend. Shopkeepers turn cold; the family orthodontist refuses to see Amanda. The brace removal has to be done by a more compassionate colleague, wearing a double pair of rubber gloves.

Hoffman omits no imaginable agony or resilience, little or big. And she discovers some that only a novelist of her sensibility could imagine.

But this brings us back to the fundamental difficulty of “At Risk.” In a way, to write a novel about AIDS can be like writing a novel about measles, floods or the shortfalls of tractor production in the Urals. Novels are not really concerned with such things, except for the cost of propaganda, instruction or calculated heart-wrenching; They are concerned with a people who live among such things.

Certainly there are people in “At Risk,” and they have the power, at times, both to wrench the heart and to instruct. But they seem to be invented, assigned and directed for the purpose of dramatizing the problem that Hoffman has given them. They exist in order to serve as tests, not of themselves, but of a terrifying human and social calamity.