What about :
"Is Múinteoir é"?
"Múinteoir atá ionat"?
They are all correct, yes. This one here stresses the fact that he's a teacher. Unlike English, Irish doesn't stress words to emphasize them; instead, it rearranges them. So what you have here is the equivalent to "He's a teacher."
Presumably, Duo marks all 3 as correct? - as there's no indication of empashis in the question.
Wouldn't the last mean "You are a teacher"?
Duo marked 'he is a teacher' as incorrect.
This is an Enlish to Irish exercise, "He is a teacher" is incorrect, you need Is múinteoir é or Múinteoir is ea é, or even Tá sé ina mhúinteoir.