My son had this dichotomy too. Exactly as you describe. We had a rich and loving home life, and he was a compliant child at home. Until he began to act up at around 15 but it was not serious.
My son was diagnosed as ADHD in response to his behavior in preschool actually. (There were way more things going on with him due to his early history, than with your son.) In retrospect I think that a lot of what looked like ADHD was anxiety. He would get anxious in school. There was never ever any desire to act out or to non-conform, but he would be triggered, and lack the resources to calm himself. When he was placed in a more structured and safe environment, he was able to contain his behavior and the acting out stopped. The thing is that the school district fought us and eventually won, and we lost that placement. What these kids often need is quite expensive for the school district. But my son got the services he needed for a time. And when we lost them we left that district.
I guess what I am saying is that there is merit to your desire to work with the school as long as they are working with you. There are risks either way. I do understand. What I could not stand is my son being scapegoated, when I knew he could not do better without help. Help to which he was legitimately entitled.
You see some of these so-called diagnoses like ODD or even ADHD are labels that are given to classify and describe behaviors. They have nothing at all to do with what is going on internally with these kids. The same behaviors can be motivated by vastly different internal dynamics. I would bet that your son has little or no oppositional or defiance in him, beyond the normal degree of a child of that age. But he is propelled by his feelings to be triggered in certain ways and it is experienced and described by the adults as being oppositional or defiant. In this sense the diagnosis is in THEM, about them, not him.
He may be anxious. He may be sensitive. He may need more of a container, more support due to sensitivity, more structure, etc. He may for reasons yet known become overstimulated. None of these things should be treated by disciplining him. Of course bad behavior should not be tolerated. But at what point is the intervention? Before the behavior happens or after.