The teacher reading this passage writes "S/V" in the margin in reference to sentence (12). What is the teacher trying to tell the writer?

In writing S/V, the teacher is telling the student to revise the sentence's subject/verb agreement.

If the student wrote a sentence in which the subject and verb did not agree with one another (for example, writing: "He were not at home"), then the teacher would indicate an error in subject/verb agreement.