MI stands for Mental Illness... but keep in mind, that not all MI dxes are disasterous... and even some of the serious ones, can be managed (we have some parents on this board with forms of MI). Depression and Anxiety are forms of MI that we are more familiar with...
But I wouldn't go assuming MI for now... chances of something developmental are higher.
ADHD "plus" would be my starting point.
Your son, for example. That ODD behavior at home? Might be burn-out from holding it together all day at school. Trust me - finding where those mental drains are, is NOT easy, because school doesn't get it. But... it will be there. Auditory Processing Disorders (APD), for example - fighting to listen, fighting to make out the words... means there isn't enough brainpower to process what you hear, OR it takes so much effort that burnout sets in. Not that you've described Auditory Processing Disorders (APD)-type symptoms directly, but... it often goes with ADHD. And, in some kids, Auditory Processing Disorders (APD) presents the same symptoms (not paying attention, fidgetty, etc.) as ADHD.
There's other hidden stuff, too - like sleep disorders. Kids who are getting enough time in bed but not enough quality sleep can present all sorts of behavior problems. Get the fam. doctor. to rule out medical challenges...
Just some ideas.