I think 6th grade is still too young to let them fly solo on grades and assignments like this. Forgotten lunch? Yes, let them starve, but this is more complicated than that. Our middle school stresses that parents still need to be involved and keeping an eye on their kids' work through middle school. I'm not saying you should try to patch everything up for him, but I would want to understand from the teacher what happened, whether C's understanding is correct/incorrect and get the teacher's advice on how to prevent it from happening in the future. THEN you can punish him appropriately.
