Also "native speaker", is the key word for be teacher of one language, the magical word for look for language teachers. Not only the necessary condition, also sufficient condition.
But really, althought I´m a native speaker of spanish, I don´t think that I can be a teacher.
For be a language teacher, if necessary a good knowledge about the language (grammar...) and the experience for teach (pedagogy, methods) that for a example a degree in philolology can give you.
For my own experience, I was tried to learn japanese in the university classes, and the teacher was only a native speaker, a japanese student of economy teaching japanese. Was very funny (sometimes.....) asked him things about why we can not say this in this
case, or why in this case we can not use this word... the explanation always was ¨I use this, and my friends, and my family, and they can understand me
Sometime who speaks like a native has a better knowledge of the grammar and spelling than a native speaker.
Maybe this is a different, who speaks like a native must to know the grammar, learn from books, and for the native speaker is not necessary have a explicit knowledge about the grammar, they learn the grammar speaking in their society.