If you're a salutatorian, can the school force you to make a speech even when you don't want to? Can they exclude you from the graduation ceremony?

First of all, congratulations !!  second, I don't know if they can force you or not but you should !  It is an accomplishment to be proud of, and don't let a fear of public speaking or some other reason deprive you of the opportunity to be recognized for the accomplishment.  If nothing else, stand up and thank the people that helped you along the way, and if no one did, stand up and say "I did this on my own".