This is what i did, pretty cheesy but works well.
I placed a poster on the front of her porch so when she got up to go to school she would see the sign.
It read "Will you rock with me to prom? bring your answer to school today" Under the yes section of the poster was a tiny pebble and under the no was fairly bring boulder. So in order for her to say no she had to bring the huge boulder to school.
Like I said, really cheesy but it works