The agony of Alice / Phyllis Reynolds Naylor.

Enhanced descriptions from Syndetics:

Life, Alice McKinley feels, is just one big embarrassment. Here she is, about to be a teenager and she doesn't know how. It's worse for her than for anyone else, she believes, because she has no role model. Her mother has been dead for years. Help and advice can only come from her father, manager of a music store, and her nineteen-year-old brother, who is a slob. What do they know about being a teen age girl?

What she needs, Alice decides, is a gorgeous woman who does everything right, as a roadmap, so to speak. If only she finds herself, when school begins, in the classroom of the beautiful sixth-grade teacher, Miss Cole, her troubles will be over. Unfortunately, she draws the homely, pear-shaped Mrs. Plotkin. One of Mrs. Plotkin's first assignments is for each member of the class to keep a journal of their thoughts and feelings. Alice calls hers "The Agony of Alice," and in it she records all the embarrassing things that happen to her.

Through the school year, Alice has lots to record. She also comes to know the lovely Miss Cole, as well as Mrs. Plotkin. And she meets an aunt and a female cousin whom she has not really known before. Out of all this, to her amazement, comes a role model -- one that she would never have accepted before she made a few very important discoveries on her own, things no roadmap could have shown her. Alice moves on, ready to be a wise teenager.