So I know many people, including me until just a while ago, have been having a problem with not progressing beyond Student of Light/Darkness. I finally went and dug into the event file for it and got my answer!

To get to the next step in Light, you need two traits: "Kind" and "Strong". After the becoming a student an event will normally fire at chance 70, giving you the Kind trait and removing Cruel and Deceitful. After that, an event will fire at chance 60 that'll give you the Strong trait.

HOWEVER!

The Kind event WILL NOT fire if you already have Strong! I have no idea why this is (maybe there's more context in the event description, I haven't seen it), but it makes you have to rely on one of the very few events that give you kind if you don't already have it.
...Personally, I just used the console to fire event 1017 because my character was 35 at that point. The event lets you choose between kind and cruel (though it's not a guaranteed chance, like most events).

Likewise, a student of dark needs the Cruel trait but the event that gives it to you WILL NOT fire if you already have strong.

Hopes this helps people out, maybe pin this to reduce the stream of people asking.