Answer: was
You would fill in the blank with the word was. Since it is your teacher, we can assume that they are already born, meaning the time of their birth was in the past. We would not use the word, were, because it is singular and we would not use the word is, since that would not be proper grammar.
This means (was) is the best word to add to the blank.
My teacher was born in Japan.
I hope this helped & Good Luck <3!!!