I'm on my 23rd year as a teacher and I always see many comments about teachers on here so here are my 2 cents (for whatever it's worth).
There are 2 problems as far as I'm concerned (at least here in Ca.). One is that we have a union that protects poor teachers and rewards teachers based on years served only rather than a combination of performance and other things that actually make sense. The other thing is that parents do not hold their own kids accountable. Homework is almost never done, studying is almost a thing of the past. Discipline is almost not there entirely.
Just speaking from my own experience. It's not always true but it is way too often.