Ideally, the assignments and instructions would be emailed to him by teachers or posted online. My son just finished 8th grade and has had the same issues. He requires very detailed instructions with clear expectations. For example, when the math teacher says, “Show your work,” my son thinks that only means the work he had to do on paper, not the work he does in his head. So, the instruction for him needs to be something like, “Write down every step you do in your head and the steps you do on paper to show how you got from the problem given to the final answer.”
He was doing so many assignments 2 and 3 times because they were getting lost or forgotten, not to mention lots of makeup work. It was ruining all his time outside of school, as well as our family relationships. All because teachers wouldn’t check his agenda and review assignments with him at the end of each class (despite being in his IEP). I finally put my foot down and demanded that he be allowed to bring an iPad for school and use it to go almost entirely paperless (I had been asking nicely for several years). They agreed and it has helped a great deal. It’s still a struggle, because the ipad has to be used and used well, but it’s a lot better.
Here’s more on high school success for ADHD:
ADDitude Community Moderator, Author & Mentor on Parenting ADHD, Mom to teen w/ ADHD, LDs, and autism