When I was in middle school (roughly ages 10–13 years old) in the US in the early 1970s, they combined English—or what might now be called language arts—with social studies into a single class that was called Core. Even (or particularly?) as adolescents we found the name curious. Nor could we have formulated a decent guess as to why one might have collected what seemed such heterogeneous subject matter into a single class. As far as I have been able to gather, this was a fairly idiosyncratic quirk localized to our county in Maryland, or even just to our school.
Although I would love to know the pedagogical motivation for this practice, that question would be off topic here. So I limit myself to the terminological facets. How widespread (synchronically or diachronically) is/was the label Core for such an amalgam of topics and what is/was the thinking behind the choice of label? By the time our kids hit that same middle school (two and a half decades later) it seemed the authorities had come to their senses because that material had been split back apart into two classes with discipline-specific names.
I presume that the name Core has to do with someone’s conception of what constitutes the core of the curriculum critical to the formation of young minds, so—roughly speaking—an education school’s dead-earnest analog of The Three R’s. But that would reflect a greatly impoverished notion of What Kids Today Need To Master compared to, say, the later-arising Common Core.