In Zen, the story that illustrates the meaning of sangha may be a good one for defining a teacher as well. The story goes: A potato farmer harvests his crop, sticks the spuds in a burlap bag and tosses them into a nearby stream. The current rubs the potatoes together and removes excess dirt.
Everyone is in a burlap bag...
All the time.
With or without a "teacher," there is no lack of current.