А кому нужно написать письмо? или надо просто обозначить проблему? Не совсем понятно задание. Вот какую проблему можно описать.
Hello,
I have a big problem and I need your advice. On Saturday my friends are going to the concert of my favorite band. But my mum doesn't let me go with them, because I got a bad mark at school yesterday. She says that I shoulld study instead of going to the concerts. She is so angry with me. But I want to go so much! What should I do?