My state has a telecommunications network that was responsible for bringing the Internet to schools and libraries in the 90s. As a result many of these institutions were assigned domain names under ia.us, which the network controls on behalf of the state. The state government gets the state.ia.us subdomain, libraries got their own second-level subdomain under lib.ia.us, schools under k12.ia.us (private schools under another level pvt.k12.ia.us, although their website now lists that as pvtk12.ia.us; my elementary school domain of the first form still resolves), community colleges cc.ia.us and so on. I didn't know better at the time and assumed the whole US was organized that way. In any case no one liked having johnd@excelsior.pvt.k12.ia.us as their email address so most of the schools bought a second .net or .org domain.

I know my high school moved off the ICN T1 service in the early 2000s, but it looks like the domain records are still maintained, as the old address still resolves correctly.

Edit: see EvanAnderson below I didn't realize this was ~formalized as an RFC and actually was relatively standard across states, I assume for the same reasons very few public entities were using these hierarchal addresses as their primary by the time I really got online in the mid 2000s.