I'm not trying to generalize, but having done informal follow-up on a few local incidents near me that hit the newspapers...a lot of school districts don't have the funding to hire enough teachers. Average class-teacher ratio in U. S. public schools is something like 30-35 kids to one teacher, I think; you get a school with 25 kids to a teacher and that's considered great. I don't remember exactly, but I believe class-teacher ratios in other countries are much lower.