Here is how I see it.

You work hard. Study. Do everything exactly right.

The teacher gives you a D.

The teacher gives their son an A.

The other students in the class realize they can't get an A no matter what they do. So they are going to kick the teacher's ass.

The teacher gives EVERYONE D's.

The other students don't like having D's. And not being able to get A's. But as long as they don't get F's, they are willing to sit and wait for things to get better.

Meanwhile, you work harder and harder and harder, trying to get that A.

And you hate all those other students for doing just as well/bad as you!

You should be hating the teacher.