I know that my experience was most likely more idyllic than most, but which outgrowths of modern public education (active shooter drills, windowless rooms, zero-tolerance policies?) make you hold it in such low regard?

People describing their school lives in modern Western countries will tell you completely different stories. From almost idyllic to hells of abuse (even sexual) and bullying that made them consider suicide before the age of twelve.

If we judge schools by the average, they are much better than the Spartan agoge, but if we judge them by the worst ones, I am not sure if it is still the case.

As you say, adults only have control over superficial parts of these dynamics. I'm dubious that they can exercise more control without extensive restrictions on the youths freedoms and in turn their development. So these problems just seem inevitable wherever groups of children or teenagers meet.

Note that most of that is unique to USA education. Especially "active shooter drills".

All these seem to be uniquely US problems..

I send them to school because I am lazy. That is the opposite of caring. My younger daughter had some nasty experiences in school, up to having to call the cops because she was sexually assaulted in class.

Schools are a cesspit.

If you want to spend thousands to send your child to a private school, so that they get a mediocre, spoon fed education, with superfluous 'perks' that don't help their development, and surrounded by rich kids who's privilege will rub off on them, that's fine by me. But the only goof justification is if you don't have a good public school as an alternative. Otherwise you're wasting your money.