Getting Blamed for Your Grade when It’s Your Teacher’s Fault

So I know a lot of times it’s just the student not trying and not doing their homework, but some teachers just aren’t very good at their jobs. I’m horrible at geometry, and it doesn’t help that my teacher won’t explain what I did wrong on tests and assignments, only calls on like two people, and has a voice that sounds like it should be on a sleep hypnosis CD. But parents always refuse to believe that teachers affect your grades, even though they do.