This is incidentally a weakness in the US education system. You split schools into elementary-middle-high (for no especial reason) then treat it all as 'grade school'.
In every European country I've lived in schools are generally split into primary and secondary. Primary school teachers are general educators whose primary skill is teaching but who do not need advanced knowledge in any particular subject. They usually have the same class all day, which gives them a deep insight into students' progrewss (although from a kid's point of view, if you don't get on with your teacher then school may suck). In secondary school teachers may teach more than one subject but they're required to have studied their primary subject at university and done some additional study in other subjects they teach, as well as additional study in educational methods. So your math teacher has a math degree, your history teacher a history degree and so on. People who plan to teach usually develop themselves academically in two subjects, sometimes three if they're closely related or you have special experience. For example: http://www.teachingcouncil.ie/_fileupload/Registration/Gener...
California seems to be moving towards this model too: http://www.ctc.ca.gov/credentials/leaflets/cl560c.pdf
The downside of this is that if a particular teacher is sick and another has to take the class as a substitute, the substitute teacher just babysits, or comes into class with a homework assignment from the sick teacher that should be doable during the class period. The upside is that in general students are keenly aware that teachers know what they're talking about and are considerably more expert in the subject than would be possible using the assigned textbooks.
I've ranted on HN before about how destructive of educational ends the practice of using 'Teacher's Editions of textbooks with scripts and answer keys is. It degrades teaching to a branch of bullshit artistry, and students who take a subject seriously can quickly detect a lack of true expertise.
Obviously this short comment isn't meant as an accurate descriptor of the whole state of the US education system, just the legacy of the non-specialization referred to in the grandparent comment.