You mean you've tried talking to every single person in your school and they all just pretend like you're not there? I find that hard to believe. I can definitely believe that it's hard to just strike up conversations; that's hard for most people and why very few conversations get started on the bus or in elevators between strangers. For that to be true, it would imply a baseless vendetta against you perpetrated by an unusually cohesive and well-organised high school population. If nothing else, I see so many threads like this on the Escapist - if there are enough people making threads like this, there must be people in your high school or community who feel the same as you! I don't buy that high schools consist solely of people whose mission it is to make one individual an outcast.
Are you sure there aren't people you're overlooking? Perhaps people even you yourself consider beneath your notice? It's always worth being painfully honest with yourself and making sure you aren't just trying to appeal to a particular crowd and projecting your failure with them onto everyone else. Variety is key, as is perseverance. If a bunch of people just don't want to know, let them go, but make sure you don't discount people after one time when you said 'Hello' quite shyly and they probably didn't hear you, kept going and you felt terrible because you thought you'd just been totally cut dead by someone. Take a step back and reassess the situation.
Those on this thread who have already recommended joining or starting a club are right on the money. Find a club you're interested in and join up. You'll probably get to learn something new and you'll have instant friends by virtue of a shared interest. I'm assuming you comport yourself politely anyway.
Another option is to find an extracurricular activity - something outside school where you can join a group (like, I don't know, a swim club, or fencing, or soup kitchen whatever) and make friends there where you have a chance to start afresh. The friendships you make there will make you more confident, and make you more socially attractive to people at school. Even if you don't have anyone to hang around with at lunchtime at school, you can kick back with a book secure in the knowledge that you'll be seeing your friends when school lets out and you head over to the papier-mache club or wherever.
Another alternative, if you're feeling lonely at lunchtimes: get involved in your school. I don't know about how it works where you are, but my school was always looking for people to help out in the library, or the P.E. shed or whatever. You could always join a musical group - if you don't play an appropriate instrument, orchestras are perennially looking for percussionists, and it's a section where the instruments are provided and you can quickly pick up a new skill! Needless to say, there will be other people in these jobs/activities that you can make friends with.
Just keep trying, and don't lose heart. If you're open, polite, and pick your targets wisely, there's no reason you can't have as many friends as you want. Good luck! =)