Who are the main detractors of GS?

Martin Gardner and Max Black originally, and a host of (loony?) people who call GS a "cult."

What are the arguments?

Too numerous to list. The ones who call it a "cult" try to tie it to Scientology.

Gardner's and Black's arguments had more to do with what they saw as Korzybski's misunderstanding of the underpinnings of Aristotlean logic, his verbosity, and his tendency to tread into metaphysical waters.

Why isn't GS taught everywhere, but certainly at the High School level?

You're kidding, right? You don't think the public schools exist to actually teach something useful, do you? I have managed to forget most of what I learned in high school, and have not missed it one bit. All that math, science, history, etc.... None of it proved useful later in life, except from the perspective of knowing more about humanity and human knowledge. But for daily usage? Most of what they teach in schools amounts to busy work to prevent kids from going buck wild in the streets, if you ask me....