I wouldn't go so far as to say that they're horrible at their jobs, just that a lot of times, they can't do the job right due to outside circumstance. Examples are thus:
1) The schools need bigger budgets, so they take more standardized tests, and instead of getting information they might need and learning it in an accessible way, they teach the kids how to pass this test so that the school can get more money. Everything from the IOWA to the LPTs are all there to help the schools get better budgets, and in turn, tend to do more harm than good. The teachers who actually have good, fun ideas to help the kids learn are shot down because their curriculum isn't up to what the school board wants them to learn [ie-testing]. And don't get me started on the whole "What is appropriate for my child to learn in school?" argument....
2) A lot of kids just don't give a crap. Why am I gonna sit here for an hour and a half listening to Mr. Pfeiffer talk about MacBeth when Rachel and Bobby are back together, and Joe got his new car? And it's gotten worse in the last ten years. When I was in high school, we still passed notes. [God, I sound old. I only graduated back in 2002!!] Now, I'm sure that happens, but a lot of the note stuff has moved to texting, which requires a bit more attention, since it's such a rapid back and forth. And while the teacher is trying to teach the kids that DO want to learn, he has to say "Please put your phone down, Ms. Compton." or "Reggie, pick your head up off the desk."
3) I know some teachers who actually fear for their lives sometimes. There's an obvious disruptor in the classroom, and the teacher can't do anything because this kid thinks he's a badass and will attack when he gets the chance. I know a teacher who was put in the hospital because this kid was sent to the principal for disrupting the class [The kid thought it was disrespectful to call him out like that in front of his peers
]. And with the rate of kids bringing weapons and such to schools nowadays, can you really blame them for being worried?