How awful Isa. My only suggestions are switching teachers or schools or if that's not possible, one thing that helped me with DS 9 was enrolling him an online course. It seemed to make a huge difference, such that he could "endure" humdrum stuff at school, knowing he had challenging work in his course. It also made him feel Special, which in big families is sometimes hard for the 3rd sibling. He and mom spend time reviewing some chapters. It has made a big difference for him, though I think it's a lot of work and extra things for me to worry about and bug him about, make time for, etc.
