I was lucky in that at my school, when I refused to say the pledge for religious reasons I was only asked to sit quietly, which I would not consider undue coercion, however in cases where students are disciplined for their refusal to say the pledge the school is using fear of reprisal to force students to speak something they don't believe in. That is the definition of coercion, and is unfair to students with religious or political reasons to not say the pledge.
Having attended public schools with in the United States, I do not feel like the recitation of the Pledge of Allegiance constitutes undue coercion. I say this because the pledge was only recited in maybe half of all my classes/years, so it wasn't something that happened daily over 12 years of school. Secondly, if the class was doing this in a group effort, there wasn't a major requirement for you to follow along. If you remained quite and didn't cause trouble, you could just as easily not recite the pledge.