7th grader becomes a winner, Beanpole, by Barbara Park. New York: Alfred A. Knopf. 147 pp. $9.95. Ages 11 and up.

Being the tallest girl in seventh grade isn't the easiest thing in the world. But not only is Lillian Iris Pinkerton 5 feet, 6 inches tall, she's as skinny as a beanpole. This is a fact that her class won't let her forget!

One of her best friends, Drew, is short and pudgy, but she's smart. Her other good friend, Belinda, isn't very bright, but she's pretty. As far as Lillian is concerned, she's nothing but a loser, the biggest loser in the seventh grade.

For her 13th birthday, Lillian makes three wishes which she is convinced will make her a winner: ''I want to dance with a boy; I want to wear a bra; I want to make the pom-pom squad.'' One by one, it looks like each of her wishes is going to come true.

But things don't always work out the way Lillian wants them to. Instead of becoming a ''winner'' in her prescribed way, Lillian learns to laugh at herself; she also learns that she really is a winner already, in spite of her build. She even learns that there are many times when it helps to be tall.

Barbara Park is offering her readers encouragement. In this perceptive novel about growing up, she captures the feeling of what it is like to be a seventh-grade ''loser'' and shows that a place on the pom-pom squad or dancing with a boy aren't necessarily the ways to success.