First off, if your group of friends who you're trying to part with were really your friends they would try to understand what your schedule/life is like but they're obviously not. Only you know what's best for you and you've already decided what would work for you in the long run. If those friends can't support you in what you're doing then you are right for trying to ease yourself away from them. You're a very motivated person and you need others like that in your life. So far it just seems like those friends are just holding you back.
There's never an easy way to get rid of friends even if you have good reasoning. Honestly, it would probably be best if you talked to your best friend in person. Explain to her how you're feeling and tell her what you feel is going to be best for you. It will be hard on her and you should probably expect her to be upset, after all you two are best friends but you have to remember that it's you who you need to think about right now.
After talking to her about everything it would be best to just cut all contact. I don't really think you'll have a problem with that since you already have a busy schedule.
My best suggestion for you would be to try to become closer friends with the people on your team or the other freshman. Even if the girls on your team are older it doesn't mean you all can't be friends. We all need to have people there in our lives so you shouldn't just try to go on with volleyball and school without having some sort of social life. Just make sure you know how to balance everything. Don't put too much pressure on yourself because that's when things will really get rough. As I said, you can always make new friends or become better friends with people you already know. You know what you're doing with your life right now and you should try to find friends who are just as motivated from now on. You don't need people who are deliberately going to question what you're doing. You seem to be a very independent person and you may need those type of friends as well. Just remember that you can't do everything by yourself, cutting her out of your life might be good but that doesn't mean you should always do this with people. Good luck.