A high school teacher often needs a master’s degree for certification, and some schools will pay for them to earn it while they’re teaching.

High school teachers often need a master’s degree for certification, and some schools will pay for them to earn it while they’re teaching.

In the first sentence, the pronouns them and they do not agree with their antecedent, teacher. In the second sentence, the antecedent has been changed to the plural teachers, so no agreement problem exists.