Formal:
* Mentor: A wise and trusted counselor or guide.
* Preceptor: A teacher, especially one who instructs in a particular skill or art.
* Sage: A wise person, especially one who is venerated for their knowledge and experience.
* Guru: A spiritual teacher or guide, often in Eastern religions.
* Master: A highly skilled and knowledgeable person who teaches others.
Informal:
* Wise person: A simple, direct way to describe someone who is intelligent and experienced.
* Wise old soul: Emphasizes the age and wisdom of the teacher.
* Pro: Someone highly skilled in a particular field.
* Enlightened one: Implies a deep understanding of life and the world.
Figurative:
* Fountain of knowledge: A metaphor for someone who has a vast amount of knowledge.
* Beacon of wisdom: A metaphor for someone who is a source of wisdom and guidance.
* Walking encyclopedia: A metaphor for someone who has a vast amount of knowledge about many different subjects.
The best word to use will depend on the specific context and the type of wisdom the teacher possesses.