...und zwar sowohl Anfänger als auch Fortgeschrittene.
...and that's for beginners as well as advanced students. Do I have the German correct? I have been struggling over the plurals.
you wrote ...both beginners and advanced. Without knowing the context I'd say it is correct, especially the plurals. But right now it could mean all beginners and advanced people.
what you wrote in English I would translate to "... sowohl für Anfänger als auch für fortgeschrittene Schüler."
"Fortgeschrittene" is something like a fixed expression, it's not as random as e.g. "Rothaarige" (red-haired persons). Technically, it's a nominalised (and thus capitalised) Partizip Perfekt (fortschreiten = to advance -- fortgeschritten = advanced -- der Fortgeschrittene = the advanced one), I suppose (I'm not good at grammar terms, sorry), but it really functions as a "proper word" in itself, because it's very common.
What I mean to say is that when people hear the word, they instantly recognise it as meaning "advanced persons" instead of thinking, "oh, it should really read 'fortgeschrittene Schüler/Menschen', but they've left the noun out". If you add "fortgeschrittene Schüler", it sounds kind of verbose, like saying "[The proceeds of this charity concert go to] the poor people" instead of "the poor".