Not sure if this is just ridiculous Eurocentric circlejerking or a legitimate question, but I'll answer it as it seems like very few people in this thread have a clue as to wtf they're talking about.
Zero Tolerance policies stem from the observable fact that historically speaking, students were punished differently depending on their race, gender, and social class. Male, black or brown, and poor students received noticeably more severe punishments than female, white, and wealthy students for the exact same offenses. I'll illustrate this with a story from my life.
When I was in first grade, I took the bus to school. There was one other child in my class who took the same bus, a white girl by the name of Beth. One day, for whatever reason, the bus was late and we both arrived to school roughly 10 minutes after the bell rang. Beth was immediately excused for any offense - after all, punishing her for a failure on the part of school-provided transportation would be ridiculous; if anything she was the victim here. As a impoverished brown boy, however, I wasn't so lucky. I was forced to spend lunch and recess in the office for the rest of the week, where I was not permitted to read, make noise, or get out of my chair.
Although my parents didn't really give a fuck, plenty of other parents of children whose demographics put them on the shitty side of this equation tended to resent the system and complain about it. Efforts were made to convince schools to stop this discrimination, but it seemed that no matter how hard they tried administrators could neither offer mercy to brown and black poor boys nor punish wealthy white girls in any meaningful way. Thus, Zero Tolerance was born - a policy that essentially said that there would be a known punishment for every known offense and no other criteria would be considered.
Where we hit the snag is that when the people who were writing these policies were determining their punishments they seem to have envisioned all the rules being broken exclusively by poor black and brown boys, and as such the punishments tend to be humiliating, draconian, and downright harmful to the well-being of the child being punished. For those of us who grew up on the bad side of the old system this is hardly shocking - we know first-hand that sometimes you're punished for things that aren't your fault, and more often than not those punishments are fairly devastating. For those who didn't, however, it can be pretty enraging.
What irritates me is that so many people seem to think that the answer is simply to go back to the old system - which would mean that the undesirable demographics continue to suffer as we always have, but at least you don't have to see the good demographics suffering in similar ways. While I can understand that this is technically preferable to the current system, I don't understand why it's so impossible for us to understand that maybe, just maybe, these punishments are just as immoral and wrong when applied to poor black and brown boys as they are when applied to anyone else.