I had a colleague who taught AP chemistry a few years back. He had this great T-shirt:
Those who can, do.
Those who can do more, teach.
And yes I'm a teacher. I teach high school AP physics & college physics/astronomy. There are a lot of damn good teachers out there, but I will also be the first to admit there are a lot of idiots in classrooms out there as well (and I don't mean the students).