I would have them bring the slipper during dessert. i really agree with the previous poster's suggestion. Have it printed on some type of parchment looking paper in a curly, fancy script. Here's a suggestion:
Prince Charming requests your presence at the most prestigous ball of the year, to be held at
PLACE
on
FRIDAY, APRIL 23
at 9:30 pm
The honor of your presence would be most appreciated.
Then, when she finishes reading the invite, parhaps say something sweet like " I promise I'll treeat you like a Princess. You're my Cinderella and I hope you would make my wish come true and be my date for the prom. "
And then later, plot with her mom or friends to get her shoe size and give her a fabulous pair of "glass" slippers. (Ones that she'd already seen and loved)
And to really wow her, make sure you give her the most beautiful corsage and see if you can have the florist attach a little inexpensive show charm.
Sounds like you are avery romantic, sweet young man and that your would be date is a very lucky princess!!