Is this too harsh?

My 12 year old son has had terrible grades all year. So many missing assignments, and I don’t understand. He only brings home one homework packet and the teacher said the rest they do is in class and they have plenty of time to do them. So this means he’s just not doing the work in class. I asked him like what are you doing, just staring at the wall?? He says no and he doesn’t know what the missing work is. I’m fed up. School is almost over, they are in the last term now.

Would it be too harsh of a punishment if I take his phone until he gets atleast Cs? I don’t think it’s too harsh but I have a feeling his father (who I co parent with) will not follow that rule at his house. What do you guys think? I’m thinking maybe he will start doing his work.