Use the word sage for someone or something wise and judicious. Thanks to the sage advice of your friend, you didn't write your teacher an angry e-mail!
Although you might think of a wizard when you hear the word sage, really it means a wise man. Today you see it used to refer to someone who has insight in a particular field. If someone is a policy sage, he knows just what advice to give politicians to make them understand the issue and respond successfully to it. In a totally unrelated use, there is also a plant called sage that is useful in home remedies and cooking.