That certainly is true. Things have changed in that sense, and for the worse. When I started high school (1966) I remember our world geography (I don't know what the subject would be called today, maybe something like sensitivity training for landmass diversity) teacher allowed the class to have an open debate on the Vietnam War, all viewpoints allowed w/o suppression. I can only guess what might happen today if the Iraq situation were debated. Pro-US positions marginalized and rejected, one would think.
The one thing I wanted to do in high school was "fit it" with a popular group. Going against "official" doctrine was one way you wouldn't. I can imagine that is a decisive factor in today's classroom. Go against the teacher's position, and risk not only your grade but your standing before your peers. A powerful weapon to force a kid to toe the line...
For thursdays class...
My son's position on the president (Clinton), affirmative action, the military, and missile defense were among the topics he found he could not discuss in class.
We coached him on "fitting in", showing "openness" to liberal ideas, graduating. Then we applauded him as he selected a school in Alabama (Auburn) to get out of the state.