We recently discussed the problem of unqualified math teachers who are unleashed on high school students (http://machinedesign.com/article/why-johnny-cant-do-algebra-0925). Philosopher of biology Michael Ruse recently mused about the subject far more cogently than we did: http://chronicle.com/blogPost/Science-for-Science-Teachers/20510/
One interesting quote from his piece:
"In 2007 (the last year for which there are available figures) within the State of Florida 1,295 people were hired to teach mathematics. Of those, only 394 had qualifications in teaching mathematics. Within the state, 1,154 people were hired to teach science. Of these, 282 had science qualifications. In other words, and I can attest anecdotally to this at my kids' high school, most of the people being hired in Florida to teach mathematics and science aren't qualified."
You'll also find a great joke at the end:
Q: What is the first name of every economics teacher in Indiana?