The Master-Apprentice System was the original way of teaching magic to young practitioners. True to its name, it involved a master (a high level magician) training one or several apprentices over time.
With the rise of the Necromancers and their threat to the Allied Lands, the Master-Apprentice system was found wanting in its slow production of new magicians. Because of this, schools of magic, including Whitehall, were created to teach new generations of magicians at a quicker and large rate.
Hedge Witches still practice the Master-Apprentice System, with an older hedge witch taking on a younger one.