You grossly underestimate the logistics involved in hardening schools. From the sheer variety of building designs, to the size of them, some having multiple separate buildings - see parkland; while not impossible per se, I think this is a much harder task to accomplish than you think.
Take Uvalde for example - they’d done a number of the things you suggested. School was secured during the day, guard on-site, etc. shooter got in because one door got left unlocked (someone remind me, did they determine if that was an accident, lock was broke, etc?), but the shooter knew about it.
Schools built in last 20 years have too many points of entry, and ground-level classrooms have large windows that are often big enough to enter thru.
Most of the security measures you mentioned I don’t disagree with, but I’m not sure they’re always going to be plausible for a variety of reasons, some very real and some that are unfortunately political.