Out of fear, she denounced her classmate who cheated on the exam to the teacher.

denounced 是动词 denounce 的过去式。denounce 由前缀 de-（表示"完全、彻底"）和词根 nounce（来自拉丁语 nuntiare，意为"宣布"）组成，意思是"公开谴责、告发"。在句中表示"向老师举报"的行为。