A teacher is not necessarily a person, who teaches you text books only.
Each stage of life has its own teachers. There was a stage, where our parents taught us something concrete. Who taught us how to walk? Who taught us how to speak? There are many things like that, for which parents are best teachers for sure.
You can compare teachers, because they teach us relative knowledge. But parents teach us concrete and practical knowledge. So they are the best teachers, at least, for some stage of our lives.