I remember taking a course at university where the teacher bragged the first day that half the students will fail. It struck me immediately that this professor was bragging that he sucked at his job of teaching.

If I said half my patients won't live through my treatment, or half the cars I fix will crash, I'd not be the person others would seek out.

If you are so good at your job (of teaching people a subject) that a coin toss is likely to do a better job on the exam, you suck at your job.

You are a Teacher. That is what the person sitting in the class is paying you for. To teach them. You work for the students. For all the high minded ideals, you are just staff to the people sitting in front of you.