There's one girl I want to come out to but I'm not sure...

Well, if you want to tell her, then you have to know for sure that you can trust her. You wrote that you trust her, but you are not 100% sure. You should be 100% sure, if you really don't want your secret to be spread.
I know that feeling when you finally want to tell someone. I went through it as well.
Here is my advice:
1. Never, EVER, rush in.
2. Make sure you can trust her. Talk with her. Spend some time with her. Become closer friends. Eventually you'll realize if she's the right one to know.
3. If it won't work / you still won't be sure / you'll realize she is not the right person to tell, then don't do it. Take a time, and maybe you'll find better person to share your secret with. If you have a best friend you can trust, find out what he/she things about LGBTQA people. Maybe you will find out that the right person to tell is closer than you think.

This is just my personal opinion / advice.

 

I agree with the above, and also wanted to draw attention to the fact you said she was 'very vocal' about her own coming out. Now, honestly, this rang alarm bells for me. Not that she'd gossip, but you might find she'd push you to 'work out' your sexuality, want you to 'tell others'. I completely understand the need to talk to someone as you work things through, but really truly, this is something that only you will be able to understand and explore.

In the end, if you're really struggling on your own and with these support forums and need help close to home, make sure you're close to her before telling her. I'd say you need to be able to talk to her outside of school too; I know how toxic a school environment can be.

Most of all, don't panic. You are not the first, and you are not alone. Good luck and best wishes, whatever you do.