At the moment there is no such tag and basing on experience from other sites I'd say that for the time being we don't need it at PMSE.
This is the old discussion whether we should have meta tags, like homework or, in other words, study. As on most sites they decide not to introduce such tags I'd do the same here.
The reason for having such tag would be if someone wanted to filter out homework/study questions from professional ones. If you ask me, we don't have such problem now, as we barely have more than one fresh question daily on the site, so I guess most of frequent visitors skim through all of them anyways.
Maybe we're going to come back to this discussion later, when there are more questions at PMSE, but then we may be discussing whether we want such question at all. Either way for the time being I'd say that it works OK the way it works now.