You do not learn a new complex skill in days, or by just visiting a teacher once. So its all abstractions I guess
You can effectively hand-wave this and say it's possible due to magic and learning spells (fantasy setting) or matrix style knowledge upload directly to the mind (Sci fi setting). It's all about making your player suspend belief and accept it as part of the game world. Generally xp points is something like this, just accepted design for an rpg...