I remembered reading this as a child, so I thought I'd give my childhood a quick rerun. As I was reading, I found a number of things I remembered and many I'd forgotten, including the ending, which was very satisfying.
On her way to school after lunch one day, fifth-grader Elizabeth chances upon Jennifer, a new classmate, sitting in a tree. She pushes Jennifer's falling-off shoe back onto her foot and a mysterious friendship is born. Jennifer, a self-proclaimed witch, holds a mystique for the very normal Elizabeth, who eagerly agrees to be Jennifer's apprentice witch.