The students were not talking as a protest and refused to answer the teacher's questions.

not talking 是动词 talk 的否定现在进行时形式，talk 意思是"说话"，加上 not 表示"不说话"的状态。 这里用 not talking 表示学生们通过保持沉默来表达抗议，是一种非暴力不合作的表达方式。