Oh, yes. Behavior mod support is often a part of an IEP, since behavior issues are a distraction from learning. The problem is that most plans are ineffective because they use punishment/reward systems, which only work in highly controlled institutions and even then it isn’t long lasting (i.e., the “lessons” of punishment/reward are not easily internalized).
Private schools are under no obligation to deal with it (i.e., it’s easier for them to expel “problem” children). So, unless they have a particular commitment to your child, it’s on you to solve the problem. Sounds like his ADHD treatment is not working, I’d consult with your doctor about that.
Do you have behavior management strategies that work at home? Are there such issues at home in the first place?