Thirteen of (us/we) students volunteered to sing during chapel.
Thirteen of us students volunteered to sing during chapel.
Thirteen of "us" students volunteered to sing during chapel.
To find the correct pronoun to use as the subject in this sentence, we need to determine if the speaker is included in the group of students or not. If the speaker is one of the students volunteering to sing, then the pronoun "we" should be used. If the speaker is not one of the students and is referring to the group of students as a separate entity, then the pronoun "us" should be used.
Additionally, it would be helpful to know the total number of students in order to provide a more accurate pronoun. However, based on the information given, we can only determine that thirteen students volunteered, but we don't know the total number of students.