Mouthing off to a professor is like mouthing off to a police officer. Yes, you can legally do it, but its just not a good idea.
Be glad you're not in my university's English department! Last spring I was in a class where several of my classmates were having problems with the professor (has taught for more than 25 years, the last 17 with my school).
Most of the class agreed that there was a problem, but there was one particular student who was the ring leader. He was a nontraditional student and always seemed to be just a little bit off of whatever we were talking about. He would CONSTANTLY make comments (when he bothered to show up, that is) about the material, but they were wrong and he didn't like being corrected. I'm sure everyone has met someone like this guy. He was definitely impressed with his own intellect and if anyone said anything that wasn't completely praising him and how special and smart he was, he threw a grade-A hissy fit.
Well this guy failed the first three essays. Our entire grade was 4 essays which were spread over the course of the semester. He decided to fabricate this petition and circulate it through our class. I want to make sure this is right, so I just looked in my email (I saved it) and there were 25 items on the petition, every single one was a lie. Fantastical lies actually, like the professor was more than 25 minutes late more than 10 times? Seriously? I never missed a day of the class, so I know who was there and who wasn't. The class never started late. This petition guy was usually late, but our professor was never late.
Well the majority of the class signed this petition and sent it to the English Chair who I'm sure got a good laugh out of it. It was riddled with grammatical and spelling errors.
And while that particular guy failed my class (I only know this because he told EVERYONE! multiple times!) our professor is teaching 5 classes this semester, like usual. I am taking the professor again and he often jokes about when he was petitioned for a redress of grades by a gaggle of angry underclassmen. Clearly, it did not ruin his life.