Any ideas about this possible prejudice and ways to convince entrenched academicians on hiring committees that some hs teachers are, in fact, smart, academically inclined thinkers and even scholars?
Given that student retention, and teaching inadequately prepared students to do college-quality academic work are the two biggest concerns of so many colleges and universities, I'm thinking you ought to be able to market your high school teaching as great prep for a TT job. Also, I'm guessing from your moniker that you're in classics? If so, the vast majority of jobs in your field may involve at least some teaching service courses (intro to ancient cultures) for the core curriculum, and again your HS experience seems pretty applicable.
In other words, it's only a disadvantage if you slink into the interview and feel ashamed about it.