dissection of animals in science class is probably something that a lot of Unleashed members are concerned about. We're currently putting a package together to help students object to dissection, but it's still in the development stages.
Here's some tips though that you might find useful.
1) Clearly define why it is you're objecting to dissection. Maybe even write your reasons down. You can then approach your teacher and explain your objections fully. If you want some help getting your ideas together, I'm sure lots of people on the forums can give you some ideas.
2) If you're not comfortable talking directly to your teacher then get your parents to help. (Having your reasons written down will help explain to your parents too, who might be worried that you'll fall behind in class)
3) Identify some alternatives to give to your teacher. I don't mean "Sitting outside in the sun", I mean alternatives like online simulations, videos ... there's even apps!! Check this page out for some starters:
If you want more links just ask.
4) You shouldn't have to cop a lower grade for refusing to participate in dissections either. If you are threatened with this, then you need to talk to the Head of Science, or even the Principal.
No-one should be forced to participate in dissections. You've done such a great thing by refusing already. Keep at it, and I bet your classmates will soon join you!