I think the medical fields "below" the MD aren't comparable to teachers in a school. They would be more comparable perhaps to the various administrative and secretarial positions in a school

Why? Being a nurse is in general harder to be than a doctor. So is a PA.

would be more comparable perhaps to the various administrative and secretarial positions in a school. I also realize that hospitals have those sorts of workers too.

Then shouldn't we compare the actual secretarial positions to each other. I see no reason why the floor secretary shouldn't be compared to a school's secretary. If anything the job is harder due to keeping track of stuff like HIPAA

But even a rookie general practitioner will often be making more money than a multi decade tenured high school English teacher

If by rookie you mean someone that has already spent three years doing that job as a resident (plus however many he does as an apprentice). Also, length of term doesn't drastically increase your salary. The lady who has been the secretary for the hospital for 30 years is still not going to be making the wages of a principal.

Or even more than a tenured professor at a university (though the star professors certainly make a pretty cool salary).

The average general practitioner (couldn't find the salary for first years) only makes 37% more than the full professor (didn't say one with tenure). That's not counting the advantages of being a professor (far less hours) or the costs of being a physician (i.e. malpractice insurance). Furthermore, a GP isn't likely to have a substantial salary increase as time goes on, so the rookie aspect of it is really irrelevant. I don't think 37% is that big of a discrepancy between the two fields.

http://www.ehow.com/info_12134129_tenured-college-professor-make.html

http://www.healthcare-salaries.com/physicians/general-practitioner-salary-gp