Another way of saying "potentially non-lethal" is "lethal", and another way of saying "can slaughter a classroom" is "can be used to not slaughter a classroom".

These "non-lethal" weapons are more likely to get you hurt than either disengaging entirely or engaging with a deadly weapon--to wit, one of the reasons they have officers and MPs go through CS gas/mace is to show them that they remain functional and as such they should never assume that somebody they've used a non-lethal weapon on is incapacitated.