_________ (Recipient details)
Subject: Apology for getting bad grades
I hope you are doing good, I am keeping good here. I am writing this letter to inform you that my result is out, and unfortunately my grades are not good. You know that I have been trying well and hard to attain good marks in my examination, but I failed to get good marks. Also, on the day of the examination for ________ (subject), I was not feeling well. Therefore, I could not concentrate well.
I am sorry for letting you down and not getting good grades in the examination. I promise to work even harder and achieve great heights in the future.
Your loving son/daughter,
___________ (mention your name here)