How do you tell a teacher you feel that they are being unreasonable?

unreasonable in what way , they are there for a job , and you are there to learn , remember that .

You don't.  Instead, you ask the teacher why he or she has taken the action that you disagree with.  You listen carefully to the explanation, and if you still disagree, you present the reasons for your disagreement, and then you listen carefully again to the response.  If you still aren't satisfied, you go to the dean of students at your school.

"Unreasonable" means different things to different people. You need to be very specific when you tell your teacher that you feel they are being unreasonable. You speak in a normal tone of voice and pay attention to the response.

general of a question to answer

You DON'T tell the person you think s/he's being unreasonable.  You say you don't understand why you aren't getting better grades. Than let the prof explain.

If the answer is that most students are having no problems, you're toast.

Too general of a question to answer.

You don’t. What you do is complete the class and don’t take their classes again in the future. Seriously. What do you think you will accomplish? It won’t help you. Trust me. 

the same way you just told us

Stop attending Marx pushers

Confront the person and say "I feel you are being unreasonable."