Can a sophomore boy ask a junior girl to prom?

life is full of taking risks, if he liked the girl for genuine reasons other than her body or the fact that he can get an older chick, then it would be acceptable

Of course you can! All girls get a little nervous around prom time, because everyone wants to go with a date. If you're into her, and you think she's into you, you have pretty high chances of her saying yes, and it's completely fine. Just because it's prom doesn't make it any different than any other occasion. Different grades for dates are common in high school and at all high school dances. Go for it