I know it feels very awful to be dumped like this for any reason. If I were in your place I would feel the same way - something like this happened to me many years ago, and it took me 6 months to get over with.
Perhaps you should accept the idea that it's over with her, and to prove this try to keep the communication at a very formal level. I know this must be hard if you actually have to see her everyday, but on the other hand please feel lucky to be in school, the perfect place to meet a thousand other girls.