First of all, you should have your son tested by a Child and Adolescent Psychiatrists (ask for a comprehensive psychiatric evaluation) to determine if he, in fact, has high functioning autism, oppositional defiant disorder, ADHD, some combination thereof – or something else entirely. You're shooting in the dark for a resolution without a confirmed diagnosis.
Since this issue is rather emergent, I strongly suggest requesting a Functional Behavior Assessment (FBA). If the school is sending home complaints about your son's behavior -- and expecting you to do something about it -- put the ball back in their court by requesting an FBA. This will force the school to really think about your son's behavior rather than simply reacting to it.
An FBA examines what comes before problematic behavior, what the consequences are for it, what possible function the behavior could serve, and what sorts of things could be setting your son off. If a “special needs” boy or girl finds classwork too hard or a classroom too oppressive, for example, getting sent to the hallway or the principal or home could become a reward, not an effect discipline. Conducting an FBA – and writing a behavior plan based on it – is probably the best way to head off discipline problems. If teachers and other staff refuse to go along with it, you may need to do a little behavior analysis on them.