I'm sorry you're finding it hard - exams are tough enough. Is there any way you could get out of the house a lot? - i.e. instead of revising at home, just stay at school - and then go to a public library to get some peace and quiet. Then when they realise you're not often home, they might get the point that you're finding it disturbing and annoying. Also it might give them a chance to let it out at each other before you come home.
Another thing you could try is, the next time your mum criticises your dad to your face, you could just tell her, "I'm not comfortable with you saying that", and then suggest that if she has to go on about him to someone, she phones one of her friends or something? Because it's really not fair that she should insult him to you, who is his daughter. And the same with your dad.
Good luck with your exams - just remember that this is not worth risking your future for. And when you get that piece of paper in the summer, just remember that each grade is worth much more than any examiner could know - because you've overcome all these difficulties at home.
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