A teacher is one who teaches a lesson in class but could be living a different lesson outside of class. In other words they talk the talk, but they may not walk the walk.
An आचार्य Āchārya is one who talks the talk, and walks the walk. Note that in Sanskrit the word chara (चर) means to walk.
Every breath of the life of an आचार्य Āchārya is dedicated to living the complete teachings.