I think that wanting to be bigger is very much an elementary thing (I'm taller than you! I can run faster than you!). By the time she is in her teens, she may enjoy being petite. And at least it makes it clear to everyone who the older sibling is (I had a friend whose 3 years younger sister was taller than her by the time they were 12 and 15, and looked older to boot. Doesn't help with the same class issue which I agree us unfortunate and I agree can probably be handled best by sending them to different schools but ther is at least one issue I'd be confident she'll outgrow, as it were.
