Talk to the administrators at the school and see if they will let you slip in a note to her diploma holder. I know at our local high school the diplomas are prepared about a week in advance, so it would be simple to let a parent slide a little note in.
You could also send her a letter in the mail from a college she has applied to, which could contain your surprise in the form of an official letter. Just print out the school logo on the envelope or something to that effect.
Another neat thing might be to make a video of all her home videos/pictures over the years, and pop in the surprise at the end of it.