Most textbooks and schools teach desu-masu Japanese (teineigo) from the beginning.

This is a bad idea because desu-masu is not basic Japanese structure and it leads to a number of confusions about what basic Japanese structure really is. You can easily learn to put basic Japanese into desu-masu form once you have learned it. But learning that form first is learning upside-down.

This video shows exactly what goes wrong when you learn desu-masu first and how to avoid it.