Phew, this is an easy one. "Eleven" is titled so because it is about a girl's experience at school on her eleventh birthday. What's up with the title? That's up with the title. Done and done.
Except… not quite. Perhaps Rachel best states what's really up with the title when she says:
You don't feel eleven. Not right away. It takes a few days, weeks even, sometimes even months before you say Eleven when they ask you. And you don't feel smart eleven, not until you're almost twelve. That's the way it is (4).
Although it's Rachel's eleventh birthday, she's not eleven yet. She has all the years leading up to eleven inside her—years ten, nine, eight, and so on—but not eleven. As the passage above suggests, she has to live her eleventh year before she can become eleven.
So the title "Eleven" messes with your expectations a bit. We expect Rachel's story to be a coming-of-age tale about becoming eleven years old. But the story is actually about the first steps toward eleven, toward growing old and heading into adulthood. By the story's end, she has started to become eleven, but that first day is a doozy.