Well first of all I would say it depends on why he made an f. If he is just being defiant and refuses to do his work or care then
some sort of punishment is suitable. (restriction or whatever) But, if he is sincerely struggling in school and needs help with his work in my opinion he should not be punished. I think this gives the wrong message to a kid who needs help in school to punish them, I do not think this motivates a child who is struggling academically. And frankly I do not agree with the whole grading system for elementary school, I think there is too much emphasis put on grades.