If you were trying to describe more than one teacher, you would use the subject pronoun "ustedes".
This is because tú and usted are singular subject pronouns, meaning they apply to or represent one person only. However, ustedes is a plural subject pronoun, which describes more than one person (such as more than one teacher). In addition, ustedes is a formal subject pronoun, meaning it is used in situations where you are addressing an elder or someone that you respect (including a group of teachers).
Therefore, your answer is ustedes.
Hope this helps!