Final answer:
The correct sentence is: They're having a band concert and their music teacher, Robert M. Smith, is conducting.
Step-by-step explanation:
The correct sentence is: They're having a band concert and their music teacher, Robert M. Smith, is conducting. They're is a contraction of 'they are,' which is the correct form to use in this sentence. Their is used to show possession, indicating that the music teacher is associated with the students.
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