I'm gonna weigh in on this, even though I probably shouldn't. I dont' have any kids, but since I was a teenager once, I pretty much know how they think. Even though the kids that got dosed were like "Oh no, we had no idea that there was pot in those brownies", because they didnt want to get in trouble, they darn sure did know what was in the brownies. The kid that brought the brownies, was probably a known pot-head among the other kids and he probably dropped little hints that he had "special brownies" and so the other kids wanted a piece of the action. even if they had never smoked pot before, there was probably some peer pressure involved.
The kid shouldn't be expelled, but he should have to do some community service, or paint the school, or something like that. And he should know that if someone had been allergic to what was in the brownies and had a severe reaction he could have been in a lot more trouble.