What to do when your child....

I am struggling with my 13 year old in teaching time management. He is a very smart kid, has the potential to be a straight A student - but only when he cares enough to put effort into his work. This is his last year of middle school and he's blowing it. I have taken away all electronics and limited after school activities and then he starts to do well again. After some time of consistent effort shown, I will slowly give back privileges. My problem is that I hate is that punishment only seems to temporarily solve the problem, what he needs is to learn balance. He needs to learn how to prioritize and to make school top priority. Any suggestions?