The first text of Macbeth reads "school" instead of "shoal"; Lewis Theobald, an 18th-century editor, made the change because he thought that Shakespeare meant that life here on earth is only a sandbar in the river of time. However, "bank" can also mean "bench," and schoolboys sat on benches, so maybe "school" is correct, and the meaning is that life here on earth is meant to teach us the way to life after death.