Dear Sam,
I was so glad to get your letter. Thanks a lot for writing me often.
You asked me what should you do if you failed a maths exam at school. To my mind, you shouldn't hide it from your parents, because they are the one who can understand and empathize you. Trust them: just tell the truth and promise that it won't ever happen again. Anyway, nothing can't be changed. Just study a lot, and retake exam well.
Praise the Lord, I haven't failed an exam. It gives me the creeps.
I prepare this way: I study about two hours a day, so, I don't have any problems with studying. Listen to the teacher, do your homework, before the exam just repeat everything and don't worry a lot. It's the surest way to prepare for any exam, I think.
Write me soon.
Love,
___ (your name)