Complex sentence: In 1971, he became the first junior high school player to win the National High School Championship in the U.S., going on to win the invitational U.S. Junior Championship three times in 1973,1974, and 1975.
Split into simpler sentences: In 1971, he became the first junior high school student to win the National High School Championship. He went on to win three invitational U.S. Junior Championships in 1973,1974, and 1975.