Hi
I'm not going to be much help here, I'm afraid, as I can't reassure you that you aren't suffering from leukemia. However, there is obviously something wrong and I wonder if you have told your doctor all the symptoms that you wrote on here. Why not make a list and take it along with you - I've done that myself and at least you don't come away realising that you'd forgotten to tell him/her a lot of things.
Lots of other illnesses can cause fatigue - life itself, in particular. You say that you are still at school and I guess you've recently had exams, so that's all pressure which could have adverse affect on your health.
My advice is, go to the doctor, tell him/her your concerns and don't leave the surgery until you get a straight answer.
Anthea