don't waste your time
This situation seems futile, so don't bother doing anything about it. No one ever gets an A from that professor, so don't waste your time perfecting your paper for a measly B.
Don't waste your time.
You will not get anywhere with it, so don't waste time trying. Mary: Should I ask Tom if he wants to go to the convention, or is he still in a bad mood? Sally: Don't waste your time. Mary: Bad mood, huh? Jane: I'm having trouble fixing this doorknob. Mary: Don't waste your time. I've ordered a new one.