How can I stop myself from fainting?

Tomorrow I have to do a heart dissection in bio, but I'm scared that I will faint. I fainted a couple years ago when our science teacher showed us a lung, but I'm not sure if I will be better by now and I want to do the dissection. What can I do to stop myself from fainting if I start to feel like I'm going to?

i know how you feel- i fainted after touching a lung in biology and after visiting someone in hospital (argh imagine if i ever actually have to have an operation- i'm terrified).. sad thing is, biology was one of my favourite lessons and i think i'd quite like to have been a doctor if i didn't faint at the sight of blood..
the best thing to do is immediately say 'i'm going to faint' because the first time i fainted i didn't- and nobody helped me so i fell off the table i was sitting on XD
also, take off a few layers because usually i find i get really hot when i'm about to faint
i find drinking water is nice, calming- don't ask me why
sit down for a while and take some deep breaths if you feel faint, putting your head between your knees really helps.
just try not to think about it as a lung, just as a thing that you're going to cut up- maybe do it with a partner so you have support?
just try not to worry and good luck :)

Just tell the teacher before it starts, and ask if you can keep a cold water at hand. If you start to feel dizzy, drink your water and look away. Take a few deep breaths, but not so much you feel light headed.

Drink a beer, smoke a cig, have some sex, you'll do fine.

Tell your teacher in the beginning of class, she might just let you sit down and still have the credits seeing as how its not a mild issue. Keep some water near by and if you feel symptoms tell your teacher your feeling dizzy..whatever. and sit down, drink some water and most importantly, get some fresh air

Remind yourself the purpose of doing the dissection is to examine and learn from it. It is a mind set if you expect to faint you will . Expect yourself to have out grown it and be able to handle it.