I'm guessing it could be a combination of the seriousness of the infraction, as well as the experience Disney has with dealing with the school/group in resolving the situation.
Think of it this way.... You have 30 people out of 40 causing a problem, Obviously there are some issues with the group organizer's ability to screen and control the group, so maybe you don't want them sending people again.
2 kids out of 40 caught shoplifting? Not as obvious a case of Organizer's not having control, since you can't watch over everyone and sometimes stupid kids will be stupid kids.
Now.... If the group chaperones/organizers are made aware of the situation, and agree it's a serious problem and show they are going thru the steps to punish the offenders and prevent it from happening again, Why prevent them from spending money on a trip back to the park again? The organizer's obviously doing what they can and understands the seriousness.
If however Disney approaches the Chaperones/organizers and get blown off with a "so?", and get no help or cooperation with the group, It becomes obvious that they don't really care, aren't keeping an eye on what their kids are doing, and can't be trusted to take actions to make sure future groups don't cause problems. Do you really want them sending groups in the future and causing even more havoc?
So it's probably another case of a decision that's made case by case depending on the unique circumstances and situation. Is a group that had someone try shoplifting a $5 trinket going to get banned from ever sending people back for a several thousand $$ trip... probably not. Is a group who caused $10,000 in damage to Disney property going to be banned from sending people back on a future trip.... much more likely. (But again.... if the school has been sending people for 10+ years and this is the first problem, they might be allowed back because of their track record.)