I'll note again that in the examples I mentioned, the school screwed up in both cases, and the alternative would've involved my five year old being locked out and wandering until I got home, so I don't really see much in the way of overstepping. (Incompetence however....)
Depending on state law, it may not be legal to leave a 5 yr old with an older sibling, depending on that child's age. So again, the school may or may not have a choice, legally speaking, but I think this is far better than the alternative. All parents are made aware of these policies at the beginning of the year, and while emergencies happen, they come with the territory of parenting school-age children. Arrangements have to be made.
As far as checking id's, I'm not sure what they'd do if an unfamiliar non-parent adult was there.