Do you think it's bad if I put spoiled milk in my teacher's cup?
Yes that's bad because it could make the teacher very sick! Why would you do that?
I have problems with a teacher in my history class who has told others in the class she's going to fail me.
I'm sure she's not responsible for you failing. It is unacceptable for any student to put any substance whatsoever into teacher's cup. The teacher could get very sick or even die.
It wouldn't kill her and it's just a little pay back for the poor grades she's giving me.
It's wrong to try to make your teacher sick by putting something spoiled into her cup. You should ask the teacher for help to improve your grades, I'm sure she would be glad to help or give you some extra credit assignments to get your grades up.