Hello,
So, if your characters find skills they normally lack the skills or stats to use, you can buff them with gear bonuses, potions, other skills, etc. in order to get them to reach this level and then teach them these skills.
They retain the skill even after they no longer meet requirements, though, so one can get and use skills significantly earlier than likely Larian desires them to get them (i.e. I taught a level 2 knight Phoenix Dive or w/e the way of the warrior 3 skill is called).
I would suggest making the spell check for the proper levels of skills and stats when attempting to use the spell, but not deleting it if it doesn't match. Just holding it in stasis until the prereqs are once again met.
Again, it's not some kind of hardcore exploit, but it is exploiting a certain system probably unintended as is.