TELL THE TRUTH! Tell your science teacher that you don't like the idea of cutting a heart, tell him that it would be the same for you to learn about a heart with other ways like reading a textbook.
Tell him that you would like to not do this experiment, ask permission to stay in a corner so that you don't have to look at the expermient, or ask for permission to go to the library!
Teachers like their students to tell the truth, because this helps them to understand you better, if you tell your teacher the truth, i think he'll be glad to accept your excuse and let you go over with it....