He didn't fail. He made a mistake. I'm sure you have never, ever made a mistake ever in your life, but sometimes the adults mess things up too.
I think you're being way too hard on them. They are going through a divorce and they have to deal with a melodramatic bratty son on top of it?
They've forgiven you for loads of mistakes you made growing up. You think you could forgive them an oversight when they're in the middle of a difficult time in their lives.
Honestly, I'm a little disgusted by the way you're behaving, and I'm ashamed to call you my brother.
I'm very happy staying with dad, because I love him, and if there was more time before school, I'd go live with Mum for a bit, too.
I hope you grow up before the holidays. I'd like to see everyone together. If they can manage to be civil to each other, I'd like to think you can be mature enough to do the same.