Tell her something you always admired about her.
Here's a not-so-secret secret worth committing to memory: Women can sniff out a fronting male from miles away. You might be temped to fabricate a tale of unrequited love, or fess up to a "crazy crush" you had on her in the 10th grade just to seal the deal, but she'll see through your fibs. It's sleazy to get laid on a lie.
Instead, focus on delivering a specific compliment based on something you observed (and actually admired) about her in high school. Was she a gifted student? Did she kill it as Sandy when the school put on Grease? Was her style ahead of its time? Was she the fiercest field hockey player on the team? Give her a genuine compliment, and she'll be more receptive to the other things you have to say. The key word here is genuine.