What can I do to get rid of this fear and sickness?

I have a fear of fainting, well I think that’s what makes me feel sick etc. You see the other day my friend fainted in class, and the next lesson I was fine, then my friend went out because she felt sick about what we were learning (it was biology and my teacher was talking about diseases and blood) as soon as my friend went out the class, my imediate thought was fainting and I started feeling faint myself, I tried not to think about it but then I started shaking, feeling dizzy and I couldn’t concentrate on anything else. I had enough so I went to the school matron, I had a few cups of water; went to the toilet, felt fine.. I walked back into my class just five minutes before the bell went, started shaking again and felt really sick. I thought I was just feeling ill. Then I went home, after a couple of hours I felt fine, then I was on msn and this boys status said ‘I just fainted’ as soon as I saw it I got a really bad headache and felt sick. I didn’t go to school because I was so scared and sick I have never fainted before, that’s probably why I’m scared because I don’t want to go through it. I don’t want to miss every biology lesson because of the thought because I have a exam coming up and I don’t want to do rubbish in it, also I don’t want to seem pathetic to everyone in my class. I have biology again tomorrow, and I’m really scared I’ll react the same again :’( what should I do ?