Obviously the parents of the children in the huge classes won't like it, but the rest don't necessarily know or care what's going on. The government uses the teacher to student ratio to come up with an "average class size" that has no relation to the actual size of an average class and people who aren't parents of high school students don't actually realise how big some of the classes are.
And even if they do, if the alternative is *gasp* paying more taxes! they might just accept it.
And even if they do, if the alternative is *gasp* paying more taxes! they might just accept it.