I think it depends on the extent of how you knew her. If she was a friend of yours, you should say "Well jeez, you look the same as you did in grade school!" Bring the positive energy in the message. She should be excited you've gotten ahold of her regardless of how.
On the other hand, if she was just someone in your class, someone you barely or never talked to....then I think it's creepy. It makes you look like not only you watched her from afar when she was little, but that you remembered her for all of the time since, and sought her out. What I'd recommend for a girl in that situation is a message saying "Wow, so I see the name *insert girl x's name here* on my friends profile, and I'm thinking back....didn't you go to grade school with me?" and just leave it at that. She should reply positively. It makes you look like you just happened to stumble across her page and did not seek her out. (Make sure you know at least one of her friends in case she dare ask you who's page you found her on.)