Okay so youre a 6th or 7th grader...I was nonly asking so I could give you the best advice for your age and situation, I'm not going to give you the same advice Inwould as if you were in highschool.
The girl is still maturing and is at that age were she likes a boy she may act mean, b/c how else are you supposed to act? she doesn't know. Get to know the girl, have conversationg during the day. Then YOU need to ask her out yourself. Not through her friend or your friend, thats lame and she will be way more tempted to say no because thats just how it is. No one wants to be asked out through a friend. Get to know her, then go for it after you have established a sort of friendship.