As a freshman, James averaged 21 points and 6 rebounds per game for the St. Vincent-St. Mary varsity basketball team. The Fighting Irish finished the year 27-0, winning the Division III state title. As a sophomore, he averaged 25.2 points and 7.2 rebounds with 5.8 assists and 3.8 steals per game. For some home games during the season, St. Vincent-St. Mary played at the University of Akron's 5,492-seat Rhodes Arena to satisfy ticket demand from alumni, fans, and college and NBA scouts who wanted to see James play. The Fighting Irish finished the season 26-1 and repeated as state champions. For his outstanding play, James was named Ohio's Mr. Basketball and was selected to the USA Today All-USA First Team, becoming the first sophomore to do either.
Prior to the start of James's junior year, he appeared in SLAM Magazine and was lauded as possibly "the best high school basketball player in America right now." During the season, he appeared on the cover of Sports Illustrated, becoming the first high school basketball underclassman to do so. With averages of 29 points, 8.3 rebounds, 5.7 assists, and 3.3 steals per game, he was again named Ohio's Mr. Basketball and selected to the USA Today All-USA First Team, and became the first junior to win the boys' basketball Gatorade National Player of the Year Award. St. Vincent-St. Mary finished the year with a 23-4 record, ending their season with a loss in the Division II championship game. Following the loss, James seriously considered declaring for the 2002 NBA draft, unsuccessfully petitioning for an adjustment to the NBA's draft eligibility rules that required prospective players to have at least graduated from high school. During this time, James used marijuana to help cope with the stress that resulted from the constant media attention he was receiving.
During his senior year, James and the Fighting Irish traveled around the country to play a number of nationally ranked teams, including a game against Oak Hill Academy that was nationally televised on ESPN2. Time Warner Cable, looking to capitalize on James's popularity, offered St. Vincent-St. Mary's games to subscribers on a pay-per-view basis throughout the season. For the year, James averaged 31.6 points, 9.6 rebounds, 4.6 assists, and 3.4 steals per game, was named Ohio's Mr. Basketball and USA Today All-USA First Team for an unprecedented third consecutive year, and was named Gatorade National Player of the Year for the second consecutive year. He participated in three year-end high school basketball all-star games-the EA Sports Roundball Classic, the Jordan Capital Classic, and the 2003 McDonald's All-American Game-losing his NCAA eligibility and making it official he would enter the 2003 NBA draft. According to writer Ryan Jones, James left high school as "the most hyped basketball player ever".