How do i get my 5.5 yr old son to do better in school? He's easily distracted and doesn't do his ignments

He's a smart boy, great reader, good at math, but despite our emphasis on how important school is and our punishments for not doing well in school, I can't seem to get it through his head. He completes half or less thanhalf of what he's igned to do in school and gets "strikes" for inappropriate behavior such as making noises, geting distracted, misbehaving, etc.