Always let them and everyone around you know what you think. There maybe someone in there that takes everyones silence as approval.
There is something that can trump the irate minority and that is an irate majority. It's hard to do I know because those of us with more than half a brain try to get along and not make trouble.
I think I would have got up and said something like. "Is it me or does anyone else think this is left wing propaganda?" "I'm out of here." Then maybe do what Bigsky said.
As a full time high school teacher maybe I can offer you a little useful advice. If you don't like it, raise a stink. Call the administration, write a letter to editor of the local paper. Get a couple other parents that agree with your views to call the school. Raise a little little hell at the next school board meeting and PTA meeting. Guarantee you'll get results. Nothing gets administrators attention faster than a verbal P.O.'ed parent. Hey, the liberal, limp wristed, anti God crowd have been doing for years and it works. Make your voice heard, make a difference.