Hi Lily, Thanks for coming to us with your question.
I will try and offer the best advice I can, for your situation. Having a crush on a friend can be a pretty tricky situation even without the added complication of it being a friend of the same sex, when you're not sure if she's into girls or not.
You're speculating as to whether or not she is into girls or guys (Or both). The fact that she avoids answering questions about whom she likes, or if she ever has liked anyone may be indicative of her not really having thoughts or feelings of that nature. Many adults are asexual and it's not at all uncommon for girls around your age to have not yet reached the stage where they begin liking people.
There are probably at least 100 ways to find out what someone's sexual orientation is without being blunt. However, not being straightforward might end up being more awkward. I think you're best bet would be to ask her straight up, just wait until you have a proper segue in the conversation. Don't pry too much if you can tell that the topic makes her uncomfortable though.
You're probably right to be cautious about telling her how you feel. I'm sure you'd hate to lose her as a friend or even worse, start a chain reaction that breaks up a whole group of friends. Let's face it though; middle school drama is for the most part, unavoidable. I can understand how difficult it must be to keep this under your hat and I can certainly relate from my own experiences. These situations can often lead us to make poor decisions so try and keep a cool head.
If you have another friend that you really trust, I would confide in them and perhaps even contract this person to help you get some of the information you seek out of this girl you like. Sneaky? Yes. Immature? Sure, but all's fair in love and middle school drama. Your walking a tightrope here but everything will work out for the best in some form or another as long as you try and remain conscious of people's feelings. Good luck and feel free to write us back with any developments or more questions.
Philip B. for Alterheros