The big thing that happened in high school is the fact that Biff did not graduate high school. Biff was failing math in his senior year. He had many opportunities to get tutored and improve his grade, but Biff was influenced by Willy's attitude. This attitude was that Biff's athletic ability and good looks were going to guarantee Biff's success. Biff has geared up to have a college scholarship, and Willy had filled his head with the idea that he (Biff) would go to college and be a great athlete and have much success. Why worry about math?
So Biff doesn't worry about math - and then he finds out he is going to fail math, and therefore flunk out of high school and not have a chance to go to college and become a sucess. Rather than accepting the consequence of his behavior, Biff rushes off to Boston to find Willy and to ask Willy to intervene with the teacher. Willy's skewed sense of reality has affected Biff, who believes that sweet talking will get the job done. However, he discovers his father's adultery in Boston, and loses interest in having his father speak for him. Biff becomes disillusioned, and drifts on from this point, confused and unfocused.
This lack of hard work and this inability to accept the consequences of his action are exactly what are holding Biff back as an adult. He is still looking for "the big idea", the "big plan" that will make him rich quick. As a result, he jumps from job to job, gaining little wealth, little independence, and little happiness. As an added detail to reinforce the lifetime of trying to get things quick, Biff was also a petty thief in high school - he steals things like footballs from the school, and thinks nothing of it. He doesn't understand that this attitude is what is holding him back - and it continues to hold him back as an adult. Even Willy finally shows Biff the truth of it, however, when he yells out, "“If you hadn’t flunked you’d’ve been set by now!” Although Willy is still thinking about the sports and the "million-dollar" ideas, it is important that he places some of the blame on Biff - showing that Biff needs to accept consequences.