I have plex at several of the schools I run.
The menu is now massive as I have things like:
Just makes it a little easier to find stuff
Thanks
Until such a feature is implemented, you might consider creating Collections for each subject (English, Geography, Science, etc.). You won’t access them from the left side menu, but go into any library and select the Collections tab at the top. As long as all audiobooks, films, and videos for “Geography” have the identical Collection name, they’ll all appear on that tab. When you add a new item, simply add the appropriate Collection tag, and it will appear in the Collection.
Note: Only Manual Collections will work this way. Smart Collections will not aggregate items from multiple libraries.
https://support.plex.tv/articles/201273953-collections/
Here’s an example I made for another post, showing audiobooks, soundtracks, and movies for the Harry Potter collection:
I agree with beckfield – unless there’s some complexity how you share those libraries, collections might be an easier way to achieve that.
If you need the subject/class based structure for sharing, you could alternatively consider using a sharling-label (English, Geography…) and let Plex dynamically create a smart collection based on that same label (1 stone, 2 birds 
)
That being said, I believe your suggestion is pretty close to what’s been requested some years before – though that never got much attention/support since then.
2021 clean-up: implemented (handling via collections) // no feedback/follow-up → abandoned)
