Why do some professors with PhDs leave their professorships to teach high school? I can surmise that some of them mightn't have gotten tenure and needed to find another job, but wouldn't these former professors be bored teaching the same (relatively basal) material yearly?

Let me know of other examples, but I was riffling some fee-paying schools and lighted upon:


Audrey Budding, of the Harvard Academy for International and Area Studies


  B.A., Swarthmore College 1982
  B.A., University of Cambridge (UK) 1984
  M.A., Harvard University 1991
  Ph.D., Harvard University 1998   

Eric Casey, former Associate Professor of Classics, Sweet Briar College. Now at Trinity School NYC.
David Gomprecht at Dalton School. 


  Gomprecht, PhD, graduated from Wesleyan University, where he majored in mathematics and physics, and then went on to receive a Ph.D. in mathematics from the University of California at Berkeley. After working as a research mathematician for a few years, David returned to his hometown of New York City, where he has now been a math specialist and private tutor for over twenty years.

Mara Naaman at Dalton School.
I have done this so perhaps can answer your question. I was a tenured full professor of physics at a state university but currently teach high school.

There are several reasons I chose to do this. 


Most importantly, I love teaching and am great at it, while I dislike the constant drudgery of research and will not be winning any Nobel Prizes. I never have to hit my head against the wall of grant competitions or peer review.
There came a time when I needed to solve the Two-Body Problem and the high school option made me far more mobile.
I get more time for myself. Classes generally require only about 5 minutes of prep time each, if that. The time between periods is sufficient. Grading takes less than 10% of the time it took at college because of the level of study and can often all be done in class while students are taking a quiz or test. When school is out, I go home and forget about work. There is no constant e-mailing from students wanting help and extensions.
My interactions with high school students are closer and more positive than those with most of my college students. I never learned most of my college students' names. In high school, I have a relationship with every student.


The down sides are minor. The pay kind of stinks compared to what I used to make, but the free time makes up for that. I sometimes have to loop in the Headmistress when making grade decisions that involve teacher's discretion, which grates on my sense of academic freedom. But I think I can deal.