I don't want this to sound offensive but does your son have any developmental disorders? Does he poop and/or pee in public when you want him to do/not do something, or is it just a random incident designed to make your life difficult? I suppose it could be manipulative behaviour but it would be the oddest I've ever heard because I struggle to figure out what the reward is that he gets from the manipulation. After all, he has to wear diapers in school, which I'd imagine would be rather humiliating for him.

If I were in your position, I would do what Gus1234U posted of being consistent, affectionate and stable. While I would certainly be tempted to do what KathyM posted, there is the risk he'll go to school smelling like poop and pee, which unfortunately would come around to bite you in the butt because many parents would view it as abuse, neglect and suggesting the possibility of failing to provide necessities of life. While that is obviously false, parents who don't know of the situation would be inclined to think that.

However, I have to compliment you on keeping a rational, calm mind in this situation, such as implementing the poop-free days chart. I hope all goes well.