Most students who drop out of high school in the United States admit they made a mistake by quitting and some say they might have stayed if classes were more challenging, according to a report released on Thursday. 　　Three out of four of the dropouts said if they could turn back the clock they would choose to stay, and eight out of ten said they now recognized that a high school diploma was important to succeed in life. Statistics show they are right. A high school dropout on average earns $9,200 less a year than a high school graduate, and about $1 million less over a lifetime than a college graduate.