She’s 12 and a Published Author : THE HALF & HALF DOG <i> by Lisa Gross (Landmark Editions: $12.95; 32 pp.</i> ; <i> full</i> -<i> color illustrations by Lisa Gross; ages 9 and up) </i>

Bret Easton Ellis wasn’t even out of college when he published “Less Than Zero.” It wasn’t Ellis’ youth so much as the incredibly jaded quality of his novel about affluent Los Angeles teen-agers that seized the imaginations of readers (and film makers). While Ellis certainly wasn’t writing a book for children or teen-agers, there is no doubt that both the book and news of Ellis’ youth circulated around high schools coast to coast.

But what kind of role model would an Ellis be for the youngster with ambitions to become a writer? To have “done it all” by the time you’re 20?

Happily, young people have other role models to emulate: their very own contemporaries who are published authors--such as 12-year-old high-school freshman Lisa Gross.

Lisa’s book, “The Half & Half Dog” was one of three winners of the second annual “National Written and Illustrated By . . . Awards Contest for Students” sponsored by Landmark Editions, a Kansas City, Mo., based publisher. Books are selected out of three different age groups of authors: 6 to 9, 10 to 13 and 14 to 19.

The other two awardees for 1987 are Dennis Vollmer, 6, for “Joshua Disobeys,” the story of a naughty baby whale who ends up beached and has to be rescued, and “Who Owns the Sun” by 14-year-old Stacy Chbosky, a poetic story about freedom. (Landmark Editions has just selected its 1988 winners.)

When a golden retriever gets into trouble with the Scottie up the street, she bears seven puppies. Three are all black, three all beige, and one--well, if you catch him on one side, he’s a perfect Scottie; if you look at him from the other side, he looks like a miniature golden retriever. He’s literally half and half!

Nobody wants the odd-looking pup. Everyone in the Robinson family keeps saying how ugly he is except little Davey, who sees it a little differently: “But he’s so unusual. Can’t we keep him?” “No way!” the parents respond and the pup is soon deposited at the dog pound.

He escapes the pound, only to struggle in the streets as a stray. One day, he sees his reflection in a mirror and recognizes what the rest of the world has been laughing at. He is strange-looking, he is different. The puppy has just about given up on ever finding a home when suddenly he is faced with the perfect opportunity to show what he’s made of. And he finds out just how special he is.

Lisa’s sophisticated illustrations are truly remarkable, considering the author is 12 years old. The last picture in the book is positively eye-popping.

Lisa knows how to tell a story that is evenly and briskly paced; the book is a real page-turner. Just when you think things look hopeless for the pup, Lisa knows how to inject humor. If the ending feels a little contrived, it is also the only ending you would ever want for this sweet and funny story.

And Lisa is addressing a theme about being accepted if you’re different--an important point for children who are often so unkind to their peers in their own desperation to meld into the group.

To enter The National Written & Illustrated By . . . Awards Contest for Students contact Landmark Editions, Inc., P.O. Box 4469, Kansas City, Mo. 64127. There is no entry fee, but the press requests that a business-size SASE be included.