I looked up some info on the magic words at MediaWiki. It turns out that SUBPAGENAME isn't working because the WikiEducator namespace does not have subpages enabled. This is not a surprise as the only namespace that enabled them by default is the main namespace. So, the question is, now, do we enable it? In order to do this, one has to go into the files to add a line of code. It can't be done from the client side.
I posted a message in the tech discussion asking if there are any reasons not to have the subpages enabled in the Wikieducator namespace and if not, could they enable it. I don't think it should be a problem since it seems to be enabled in the User and Workgroup namespaces.
Workgroup isn't actually a namespace. The "Workgroup:" bit is actually part if the title, which is why the talk page is actually at "Talk:Workgroup". This is a situation I've worked with before, and we called it a "pseudo" namespace. I'm not sure why WE has done things, but in regards to subpages, they work because the pages are actually in the main namespace, which does have them enabled.