Al and Jefferson bet their tax refund money on a college football game. That would be very difficult since taxes and refunds occur in April - four months after the college football season has ended.
Al tells Chad he was hit by a 300lb linebacker. High School linebackers aren't that big. However, a character trait of Al's is that he always greatly exaggerates a person's size.
Al tells Chad it was 20 years ago when he played his big high school football game, but this is not true. Al was in high school in the 60s not 1974 which is 20 years from the time of this episode in 1994.