If she's doing it coz she thinks it's funny, consider what she's looking for in your reaction and don't give the reaction she's looking for. I'm in year 8 (in uk, 12-13 yr olds) and me and all my friends are 12 but they can be real immature sometimes. (read the next paragraph only if you can be bothered to)
We always steal eachothers stuff then give it back like a minute later and joke about eachother but it's kinda got a little mean and immature coz today I went to a club but we don't get to eat our lunch so I sat down with some girls who I get on well with but they are not my bffs. My bffssaid that apparently I looked them straight in the eye (they have said this before so I know its a lie) and that I would have heard their names being called by a boy who was annoying them who was near me, they started having a go at me because apparently I had ignored them and sat with the other girls who they are a little jealous of. I also know that this is a lie because they told me that I sat down with one girl who was the other side of the group. I didn;t see my friends anywhere. I told them this too.
They were being real spiteful about it and I get worried real easily and they are often like this. My plan now is to figure out what reaction they want to get from it and make sure that I don't give that reaction so they are less likely to do things like that in the future. Remember, you are not the only one.
If it's not long winded things and just spiteful comments, tell her that you didn't like what she just said or if she said something like (this is just an example, it's not true at all) "you are ugly" ask her "am I really?" sounding upset. let her know how you feel when you she says things like this. If she just argues with you, sorry but she's not your true friend. I'm having doubts whether mine are.
sorry for the long comment
BE NICE TO DUCKS
You should tell them how you feel. and if that doesn't work it's time to find some new friends. You should stand up for yourself if your BFF is to scared to.
Just because no one notices you doesn't mean you are unnoticable. It just means that you are to good to be noticed
Maybe it's time you get yourself a new bestie?
Be friends with your current best friend, but how about a little distance? Don't give her too much about you that she can use against you. And don't back stab her either because there won't be any difference between her and you if you do that.