You are using prohibition's failure (prohibition, as the total banning of alcohol, not simply prohibiting it, as it currently is) as some sort of idea that effects this case. News flash, even if pot was legal, it would be restricted similar to cigarettes and alcohol and minors would still be in trouble for having it. If this kid brought a bottle of vodka, he'd have faced the same punishment from the school, similar to if he had brought a pack of smokes.
And if he had brought a bottle of water and some kid thought it was vodka? Or brought in candy cigarettes? (if those are even available anymore)
The issue here is that the kid did NOT bring any drugs to school. Was it reasonable to suspend the student due to the suspicion he had brought drugs to school? Sure, I don't think anyone's arguing against that.
If the leaf had been mj, would the punishment requiring him to attend a different school, and undergo daily searches be reasonable? Sounds fine, I don't think anyone is arguing against that.
When the leaf was determined to be completely benign, is it right for the school to pursue punishment the same as if it had been drugs?
I don't see that at all, especially not for a 11-12 year old. At that point the appropriate reaction should have been to have a discussion with the school administration, the child, and the parents; during which time it would be made very clear to the child that pretend drugs were not ok, and any other incidents of this sort would lead to serious punishment. At which point the child would resume classes.
Children do immature things, it's sort of the meaning of "child" and "immature" and the goal is to help them learn maturity. However leveling punishments that are completely out of line with infractions does not teach maturity, it teaches resentment, and fear.