My high school homeroom teacher allowed me to stay quiet during the pledge, but still made me stand up and put my hand over my heart. In 11th grade I moved to a more conservative area of the state, but oddly enough they had more students who opted to stay seated and silent, and the teachers let me do the same.
It was around that time that I started feeling like there was something wrong with all the "America's so great!" stuff we had been taught since 1st grade. I completely deprogrammed when I had the opportunity to work alongside people from all over the world, learn more about their culture, their family, become friends. I began to see the world differently, and my perspective shifted as I learned more about the consequences of American policies and interventions that they don't tell you about in the evening news.
So I guess what I'm getting at is that back then, I didn't say the pledge because part of me was beginning to question the reasons why it was (practically) compulsory. These days, I wouldn't say it because I cannot in good conscience support imperialism.