I guess it depends on what sort of person you yourself think you are
Do you think you are mean or spiteful?
Do you think you are usually a pretty nice kind of person and easy to get along with?
If it is not the first option and more like the second option, then why on earth would you want to tell someone that you don't want to be their friend anymore...?
That's so hurtful - even if it is true
My advice would be NOT to bring the whole matter to a head and tell her anything. Let her work it out for herself - over the next week or so.
Don't ignore her, but try to spend more time with your other friends.
Mix with them at recess or lunch. Invite them over after school to do homework, etc. and gradually find excuses not to hang out with her.
If she doesn't have any fun with you she will probably go and find others who she can chat with and leave you alone.