I don't see a change in formality. There is a difference in stress on word syllables and stress on words. The stresses on the first syllables of Joseph and teacher are just a part of the words.
In a normal use, one could stress the word "Joseph" or the word "teacher". Usually, it would be "teacher". That is just asking for information.
If one stresses "Is" it suggests that the speaker has doubts about that.