Dear (recipient's name),
Your son, (name), is having some trouble in his classes. In addition to some attendance problems, he is also failing his classes. He has performed poorly on several exams and his missing class contributes to his poor participation grade.
(name) has received several notices and has even spent time in detention. I know if you were fully aware of this situation that you would do everything you could to resolve the problem. That's why I've opted to write this letter. I hope you will do everything you need to do to help your son graduate. At this point, he isn't going to be able to graduate with his class without attending summer school.
Give me a call and let's see what we can work out. (name) has always been an excellent student, so this recent behavior really concerns me.
Sincerely,
(your name)