I'm an eighteen year old High Schooler. I've been in a steady, wonderful, beautiful relationship of two years with a person I've considered to be completely compatible with me. This has been the very first relationship I've ever had, given that I don't (or didn't?) believe that sharing every part of you (except sexually, so far) could be given to more than one person in your lifetime.

Now, here's where the contradiction comes in; Every day for the last two years I've sat next to one guy, whom I have grown a friendship with that is beyond compare to any other one I've ever had. He's not a person that would be in a monogamous relationship, and I don't even know if he'd be willing to be in a relationship with me. For a very long time my amorous feelings have been intensifying, and the tension between us is heartbreaking, the more so for me, because of my current relationship. I feel two different things at the possibility of a relationship of this kind, joy and misery. Joy, because he's a marvelous person, flaws and all, and misery because that would mean either lying to my current and very supportive boyfriend, or leaving him. Leaving him would hurt, but so would not doing something about my other love. What the HELL do I do?

Being a teen is all about confusion. At least you are articulate and know you are confused, so you are way ahead of a lot of others.
I think you might drop the 'one in a lifetime' theory but I'll leave that for you to decide.
Even into your 20s, opinions, likes, dislikes, plans, and desires can change on a dime. I've done it myself. I think 99.9% of us did.
If you are in some agony over the guy sitting next to you, it's possible he is feeling something for you that he is hiding equally well. You just won't know until you try, will you? And given that you are probably both graduating any day now, wouldn't you like to know? You could put it just like that - we are both leaving soon and I just have to tell you that my heart skips a beat sitting next to you, and I wish something but I don't know what, and then walk away, and if he stops you - take it from there.

I don't (or didn't?) believe that sharing every part of you (except sexually, so far) could be given to more than one person in your lifetime.

Your situation blows this theory all out of the water, doesn't it? In truth we are all capable of having intense feelings for more than what person at a time. That's human. But we don't have to act on, or cross the lines of good behavior, because of those feelings, especially when we have no clue how another feels.

So do nothing, just change your thinking about how you manage your feelings. I have to be honest, some feelings trigger some great fantasies, but have nothing to do with reality. So base your actions on reality, not fantasy.

Feelings or not, don't be a cheater, that crosses the lines of good behavior.

You break up, stay single, and just have fun. It is obvious that being in a committed relationship is not something you need right now.

You are playing 2 guys. Break up with the other and be with the one u want or in general break up with both of them and be single. It's clear that you don't know what you want because if you did you would have already broken up with one of them.