I did not watch every teacher at every school. Nor have you. Your contention was that they work 15-18 hours a day. I said not the ones I see. Not the ones I know.
So which is it? Are they underpaid and overworked of overpaid and underworked?
So you admit that you do not see or know "all" teachers.
Please show me where I said teachers work 15-18 hours per day. I don't recall giving specific numbers, but I do contend that (1) all teachers do not work minimum hours, as you say they do, and (2) teachers can do much of their work such as planning and grading papers at home, and many teachers do put in many hours at night and on weekends doing just that. Your informal surveillance of the teachers' parking lot would not reveal that, however.
So which is it? Are they underpaid and overworked of overpaid and underworked?
Probably both. Some teachers work much harder and some are much more effective than others, but teachers are paid by their degrees and experience, not by the quality of their work. Frequently good teachers are paid much less than poor teachers.
Pay scales depend on the state and the district. That depends on the strength of the union and the cost of living in a particular area. The same teacher doing the same job would be paid much more (and I believe work fewer hours) in California than in Arkansas, for instance. On the other hand, the cost of living is much higher in some parts of California.
It's also true that teachers in some fields, such as science and math, could probably make much more in the private sector than they do teaching and than their counterparts teaching history and English could, but while teaching they make the same amount if they have the same amount of experience and the same number of degrees.
The point is, painting ALL teachers with a broad brush based on what you claim to see, especially since you are probably not seeing the entire picture, is not right. In fact, it might even fall into the category of "grossly uninformed or purposely trying to distort the issues."