How do i make my friend happy?

I am a popular girl, in fact probably the most popular girl at my school, NOT BEING VAIN ! (year 7, end of primary school) and i am in a friendship group of about 8 girls. one of them however, is kind of labelled as 'uncool.' i always feel particularly sorry for her, because the girls in my group always try to run when she comes, block her out of the circle when we talk, and not invite her to parties and get togethers. whenever this happens, i always get angry at my friends for not inviting her, being mean to her or excluding her. i do feel it is kind of my responsibility to help her stay happy, since im the only person who actually notices her pain and hurt. i dont really have a problem with that, but i cant always be that alert to identify all the bullying, and i cant always pick it up in front of everyone with all the peer pressure and keeping my friends. the thing is, my friends are such wonderful friends to me, and i know for sure i am not insecure and scared of losing them, but they are really not nice to her. i really REALLY want her to be happy, and she will be more confident for high school next year. she is not the most attractive person you have ever met, nor the thinnest, but she does have a really kind heart and personality. she is so bubbly and funny when people are nice to her. today i spoke to her and asked her if she was okay and she just burst into tears. it really broke my heart. what can i do to help her become happy while keeping my own friends !!?? :(