Well, wait it out. Sure, its probably a doomed cause (If you eer have to say "Wai it Out" When you are trying to get a girl to date you, its usually not a good idea. Especially in High School Relationships, which is what I'm guessing fits your situation) but if you like her enough, its worth the fight.
I was just in your sitution pretty much about 2 months ago. Met a nice girl at a Play i was in, and I had a terrible little crush on her.She hooked back up with an Old Ex and she wasnt happy with him, but the relationship lasted 2 months, helped her through it. 4 weeks after they broke up, I asked he out and got shot down. We're still friends though, the good thing about asking girls out is the worst thing that can happen is no, and thats usually not terrible.
So wait it out, wait for them to break up, wait about a month, maybe two, then ask her out. If she says no, thats okay. It provides alot of relief just asking a girl out after all the frustration of waiting. Also keep your eye out for other girls, maybe a wonderful girl is just around the corner for you.
To elaborate on your post: I'd go as far as to say that "to wait it out" in this situation is generally a rather
bad idea, because more often than not it seems to devolve into people leading themselves on. You know this, of course, because you alluded to it in your post. You have your act together, for which I commend you, but unfortunately a lot of people in this situation (not necessarily here, on this forum, but in general) don't seem to understand that
just because a girl wants to be friends with you doesn't necessarily mean she is interested in more. Usually (in my experience) this confusion leads to a whole boatload of trouble.
Some people seemingly become friends with someone they're romantically interested in, but isn't available in hopes that if they wait just long enough, the current relationship will break down and she'll either become available or romantically interested in them. I needn't point out that this isn't the best of bases to found a friendship on. It can be even worse, however, because in this case you're putting yourself in a position where cultivating hope becomes attractive. I.e., you start to lead yourself on there is more than is really there. From here on it's only a small step to "actively persuading her to see I'm the right partner for her", which, if she's in a relationship, spells disaster.
If you're not already friends with the person you're romantically interested in, and she's in a relationship, then (in my opinion) it would be wise to just move on. There's plenty of other girls out there that aren't in a serious relationship.
If you develop romantic feelings for someone you're already friends with, it's different, especially if he or she is already in a (serious) relationship. In this case, being with her is definately not an option. If you've been friends for years without anything happening, then chances are she's not that into you, romantically speaking.
Whether or not to tell her how you feel, is something different entirely. If you're best friends and the relationship she's in is serious, then it might not be the best idea to tell her. Even if she does understand, it will not only put a strain on your friendship but on her relationship as well.