Can't dissect or talk about blood what do i do?

Im going into biology next year(9th) and Im not sure, but there might be dissections and I really cant do them. Lat time I tried in 7th grade I passed out and hit my head and got a concussion.I have no control, im out and I barely remember the whole day. Im also extremely allergic to fermaldahide(sp?) and bleach, they tear up my lungs and Im an athlete so lungs are really important. I pass out easily at the sight or sometimes discussing blood if its not my own(weird much) and I developted a slight asthmatic attack when I was around bleach when I was younger and had to go to the hospital. Im nervous it will bring down my grade or effect my score, my last teacher let me write a HUGE essay to cover it and Ill do it again if i have to, so how can I explain my situation without crying, im a bit sensitive about my fears because people have ridiculed me for them. Should I try to explain or have my parents? how can I not cry? will my grade drop? thanx in advance