Sabrina Bouvier is either a seven-year-old girl or an eighth-grader at Stoneybrook Middle School, depending on the book.
Sabrina the Pageant-Head
In #15, Little Miss Stoneybrook...And Dawn, Sabrina is a contestant in the Little Miss Springfield pageant, competing against Claire and Margo Pike, Karen Brewer, Charlotte Johanssen, and Myriah Perkins, among others. She is slightly younger than seven-year-old Margo, but appears much older ("about twenty-five", Dawn comments).
Sabrina wears lots of make-up and is extremely well-trained in the skills of a pageant contestant. For the talent competition, she sings the song "Moon River" while wearing a long black dress and white gloves (like Audrey Hepburn in Breakfast at Tiffany's, an allusion lost on the BSC). She goes on to win the pageant and the title of Little Miss Springfield.
Sabrina the Eighth-Grader
Sabrina's next appearance is in #60, Mary Anne's Makeover. After Mary Anne gets her hair cut in a trendy style and begins wearing make-up and more fashionable clothes, she finds herself receiving the attention of classmates she'd never bonded with before. She remarks, "Sabrina Bouvier and Susan Taylor, for instance, were two girls the BSC never liked. Why? Because they wore lots of makeup and expensive clothes, and seemed snobby. Well, seemed is the important word. It turned out that Sabrina and Susan were really friendly. On Friday they had complimented me on something, out of the blue."
In the same book, during a study hall, Sabrina sits next to Mary Anne and asks her if it's true that a Stoneybrook High School boy named Carlos Mendez had asked Mary Anne to his school's Winter Dance. (Sabrina implies that she's attended the Winter Dance at least twice before.) Later, Sabrina corners Mary Anne again and informs her that the word is that she had accepted Carlos' invitation. At SMS' Winter Jamboree at the end of the book, Mary Anne and her friends see Sabrina (looking "about twenty years old") with her date, and conclude that the date is Carlos. (In #83, Stacey sees Carlos at Pizza Express and mentions as fact that he used to date Sabrina.)
According to Kristy, Sabrina once dated a teacher named Mr. Jordan. While on a date with basketball star RJ, Stacey sees Sabrina looking at her enviously. Sabrina is seen dressed as Cleopatra and dancing with Cary Retlin at the SMS Halloween Masquerade. In M#33, a student named Jim Poirier sends Sabrina a Valentine-gram (that gets copied and posted all over the school), and they are later seen holding hands.