There are a few classes in Purdue's CS undergrad program (at least as recently as a couple years ago) which leaned a lot on XINU to teach OS fundamentals. Specifically in the 400-level Operating Systems course, we had one lab toward the end which reached into Linux for some basic stuff, but otherwise it was pretty much all XINU; playing with memory allocation, writing a new process scheduler, stuff like that. Purdue maintains a lab of XINU computers, which I believe are just Linksys routers of some kind, which students can log in to from anywhere to complete lab assignments, including a scheduling hypervisor which ensures that we weren't stepping on each others toes given there were only ~30 machines.