It really depends on your current relationship with this girl. I mean some of the above suggestions are "cute" yet may be a bit extreme. I doubt you want to go overboard with your message.
If the girl has feelings for you, a simply conversation that leads to asking her out for prom would just do the trick. Meaning don't abruptly say it, just be composed (unless you really want to suprise her). You know what? Some girls might like abruptness, just don't do something silly like throw a pokeball at her head (preferably).
If she accepts your invitation. Then it would probably be a better time to start planning up a further suprise, probably something like mentioned above, although nothing too overboard, unless she's the type. Simple is good, just showing interest in what she likes shows you care. Oh, and there is a couple of new pokemon games coming out if you didn't know, just a heads up.
Good luck.
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