I bet you're trolling. At any rate, I'm surprised the mil folks haven't chimed in already. The term came from the military. The RAG/FTU/RTU (depending on the branch of service), which are the squadrons responsible for and whose primary mission it is to conduct initial and upgrade training for the major weapon systems in question, get called the "schoolhouse" for the respective airframe. As military culture flows into the employment rolls of the airlines, so does the vernacular. Not all that cosmic really.

Nobody on this side would ever consider such a moniker remotely derogatory. I was raised part 61 before my military days and I can't say I've met a civilian pilot who considers such a use derogatory either.