He worried his roommate might tell him out to the school about cheating on the exam.

tell 在这里的意思是"告发、举报"，指向权威机构或相关人员报告某人的不当行为。 tell out 是一个短语动词，由 tell（告诉）和 out（出来）组成，表示"揭露、告发"的意思，特指向外界透露秘密或不当行为。