That kid was rude and name calling is a form of bullying. Take this issue up with your school administrators.
When I was in schools we didn't have a GSA or a day of silence. We had Christian Athletes Association that prayed before every practice and every morning before school in the middle of the courtyard. Why were they shoving that in my face?! Oh, wait...they were supporting one another and their connection with their beliefs. It had nothing to do with me. Just like the day of silence has nothing to do with you if you choose not to participate. The kids participating are gaining support from each other that may be severely needed by some of them.
My advice? Report the bully, burn the letter to burn off your anger and go on about your life being nice and participating in what you see fit for yourself. Though it is not your cup of tea, this event could pull someone struggling with their identity back from the brink of self destruction.

*~*moonlitwish*~*