I'm someone who is expected by my teachers to do well in every single subject I take, and I want to do well, and please them, too.
The truth of the matter is, working hard enough to please everybody is actually hugely stressful and there's almost too much pressure to handle, particularly when, as a teenager, I also want to have a social life. All in all, it's been very hard on me. I've made myself ill, I've spent many a time crying over something completely stupid about my schoolwork because it got too much.
I expect this girl you talk about is going through a lot of the same stuff I did (depending on yours and her age. Because, lets face it normal schoolwork is not as crucial as the GCSEs I'm taking). Getting perfect test scores is a heck of a lot harder than it looks, and sometimes it's just not worth all the stress. Don't envy her as much as you do. And, if you really want to be as good as her, you can't just expect something to happen, you have to work darn hard to get there.
But, honestly, if I could go back, I wouldn't have put myself through all that. My best advice is not to compare yourself to anyone else. There will always be somwbody better than you at one thing, but you will be much better than others at something else. It's about pleasing yourself, living up to your own expectations, not the expectations placed upon somebody else.