Wouldn't this be teachress?
Can't say I have ever heard that where I am, we just use teacher, regardless of gender.
Based on a quick look on wiktionary it claims it to be 'archaic except in India' however, someone may know more about its usage.
Is "ego sum magistram" also correct?
No, the forms of "to be" - here "am" - requires nominative case for both the subject and the coupled word.