(I'm swedish, not american, yada yada...)
Aside from a few outliers I've always had excellent teachers, especially in senior high. When I complained to the math teacher that I had difficulty solving problems (l discovered later that I had ADD, simply couldn't keep the numbers in my head for long enough) he borrowed me his copy of "how to solve it" by Pólya. At the first chemistry class the teacher told us an anecdote about how he'd taught students to make fireworks and how a few students later had tried making a batch for new years eve - killing one and maiming the others. So, no fireworks. One of my phys-ed teachers was an ex-jaeger, the other a former acrobat and the curriculum actually included some theory as to why and how you should do things. Shop class guy was a carpenter by trade. My electronics teacher was a former EE (though unfortunately I think he was a bit schizophrenic, he sometimes gave us quizzes on alchemy... nevertheless, he seemed to know his electronics well...), etc.
Even those teachers who didn't have professional or academic backgrounds had at least an interest in teaching the students actual knowledge about whatever they where teaching. Looking back, I think you could say that the only bad teachers where those who didn't have knowledge about the field they where teaching, regardless of being qualified teachers. Excepting those who had personality disorders and such - one female teacher in middle school had "real aspergers" and couldn't deal with people. She once threw a book in the head of poor Johnny (his actual name) beacuse he kept making such noise. Later, poor Johnny was assaulted by the librarian because she had forgotten to take her antipsychotics. Poor Johnny grew up to be a manual laborer AFAIK. Not that it matters here, because professional manual laborers have pretty high salaries.