Modified days (early pick ups, late drop offs) were a common strategy for dealing with behaviour at the school I previously taught at. It worked really well for the reasons you said, and, if you talk to your son's teacher, she can either restructure the days so nothing new is taught in the last hour, specialist lessons can be changed so he doesn't miss out or missed work is sent home. I think it's a great idea. Your mum might not approve in the short term, but if it starts to show results (improved behaviour, better learning outcomes, less behaviour intervention - getting in trouble - then surely she'll have no reason to disapprove?

Talk to the teacher and admin and get an official plan and monitoring in place so that improvements can be tracked and rewarded too. Good luck and good on you for being so supportive of your son.