Nervous about him going back to school

My foster son was expelled from his last school a few days before school ended. He gets angry and will get very violent if people are mean to him. He has hit female and male students. Female and male teachers. Odd thing is he's never gotten violent with me, my husband, or my kids. A girl was saying something to him about his parents not loving him so he attacked her and broke 2 of her fingers. That was a horrible way to start off the summer. He is 15. Going to be a 10th grader. All summer long we've been working in therapy and as the first day of school gets closer I've been going over our plan with him. We've gone over his coping skills several times. I'll be bringing in a note from his psychologist saying he can step out and take a break if he needs to. I will be sending him to school with his father's jacket for comfort. With a stuffed animal he can keep in his backpack that smells like his dad. And as an absolute last resort if he feels he's going to get violent to step away and call me and I will just pick him up from school. Before he got to us he was expelled from 2 other schools. I've been building him up all summer and we've been preparing how to act when people are cruel. I just really hope we can have a good school year. I'm so worried.