A letter was written about a child denied graduation rites because they were home school. The writer was upset with the school because participation on school sports qualifies for a graduation ceremony.
It is not the school you should be disappointed with but your grandchild's parents. They opted him out of the school system.
Your grandson did not work with his peers during the school years, he did his work apart and alone from them. He did not work with the teachers or participate in class discussions or social events at the school. He got to play with them.
If parents want their child not to be a part of the school system academics and only take the extra curricular activities then fine. I can understand that and it is your choice as a parent. The school however does not owe your child a diploma nor a graduation ceremony.
In my opinion if you think the school does a lousy job of educating your child then why would you want a diploma and graduation ceremony from them. If you don't want the school educating your child then the school should not be teaching them sports or any other extra curricular activity.