Freshman season On May 29, 2019, James transferred to
Sierra Canyon School, a
private K–12 school in
Chatsworth, Los Angeles, for his
freshman year of high school. He joined the school with his brother
Bryce and incoming senior
Zaire Wade, son of
Dwyane Wade, a longtime teammate of James’s father. Sierra Canyon's basketball program, under head coach Andre Chevalier, was one of the best in the nation and won the previous two
California Interscholastic Federation (CIF) Open Division state titles. The 2019–20 team was led by five-star recruits
Brandon Boston Jr. and
Ziaire Williams, who transferred there for their senior years. Before the season,
ESPN announced that its networks would air 15 of the team's games. Despite heavy media attention, James's father barred him from being interviewed. in 2019On November 21, 2019, James made his high school debut, scoring 10 points off the bench in a 91–44 win over
Montgomery High School. On December 14, he scored 15 points, including a go-ahead layup, and was named the game's
most valuable player (MVP) in a 59–56 victory over his father's alma mater,
St. Vincent–St. Mary High School. On January 15, 2020, James scored a season-high 17 points in a 117–62 win over
Viewpoint School. Five days later, during a Hoophall Classic game against
Paul VI Catholic High School, a spectator threw an orange peel at James, briefly stopping play. A clip of the incident went
viral, drawing criticism from James’s father, and the spectator apologized. Sierra Canyon went 30–4 on the season and reached the Open Division state championship game, which was canceled due to the
COVID-19 pandemic. As a freshman, James averaged 4.1 points and 15 minutes per game, and was the team's only player to appear in all 34 games. In February 2021, James
tore his meniscus at practice and underwent surgery. Due to the COVID-19 pandemic, he had not played any official games for Sierra Canyon in the 2020–21 season, which was set to begin later that year. After missing most of the season, James returned against
Centennial High School in the
CIF Southern Section Open Division final on June 11. He scored seven points in an 80–72 loss. Sierra Canyon, led by junior
Amari Bailey, finished the season with a 16–2 record.
Junior season In James's junior season, his Sierra Canyon team was led by seniors Amari Bailey and Kijani Wright. On December 4, 2021, he scored a game-high 19 points in a 71–53 win over St. Vincent–St. Mary High School in the Chosen-1's Invitational at
Staples Center, as Bailey was sidelined with an ankle injury. His team was upset by
Harvard-Westlake School in the CIF Southern Section Open Division semifinals, where he scored four points. Sierra Canyon reached the Open Division regional final, losing to Centennial High School, 83–59, and James was held scoreless while playing through hip soreness. The team ended the season with a 26–5 record. As a junior, James averaged 8.8 points, 3.3 rebounds, 2.8 assists and 1.9 steals per game in 29 appearances.
Senior season In his senior season, James stepped into a leading role and was joined on the team by his younger brother,
Bryce. In August 2022, he played alongside his Sierra Canyon teammates for CBC in three
exhibition games in Europe. In his second game, James led all scorers with 25 points in a 97–85 loss to U18 French Select in
Paris. On October 29, James and his team were evacuated from a preseason game against the Hyattsville Stags, a team representing
DeMatha Catholic High School, after a fight broke out in the stands and a fan reported seeing a gun. Although no firearms were found, the game was not resumed. After missing the first two regular season games with an illness, James made his senior debut on November 26, recording 13 points, four assists and three rebounds in a 63–60 overtime loss to Rancho Christian School. In his first home game on November 30, he scored 25 points in a 77–61 win over
Crossroads School. In January 2023, James missed four games with a knee injury and scored 19 points in his return on February 1 as his team lost to
Notre Dame High School, 88–61, at the
Mission League semifinals. Sierra Canyon was relegated from the CIF Open Division to Division I after its worst performance in the regular season and Southern Section Open Division playoffs in the tenure of Andre Chevalier. James scored 10 points in an 80–68 season-ending loss to Notre Dame High School at the Division I regional final. As a senior, he averaged 14.2 points, 5.5 rebounds, 2.4 assists and 1.8 steals per game, leading his team to a 23–11 record. He was selected to play in the
McDonald's All-American Game and was named to the United States team for the
Nike Hoop Summit. James scored 15 points, making five three-pointers which tied the record for most made three-pointers in the event ever while dishing out 4 assists and 2 steals, in a 109–106 loss at the McDonald's All-American Game and finished second in the event's dunk contest. In the Nike Hoop Summit, he scored 11 points, 4 rebounds, a steal and a block as along with hitting the go ahead free-throws as his team won, 90–84.
Recruiting By 2015, James was receiving scholarship offers and letters of interest from
college basketball programs, according to his father, who commented, "It should be a violation. You shouldn't be
recruiting 10-year-old kids." In 2016, ESPN reported that James held offers from
Duke and
Kentucky. Entering high school, analysts viewed him a high major
NCAA Division I prospect. He first appeared in recruiting rankings before his sophomore season, and
247Sports and ESPN rated him a four-star recruit and a top-30 player in the 2023 class. Early in his junior season coming off a season ending injury, James fell to 52nd and 49th in rankings by 247Sports and ESPN, respectively, and he dropped 31 spots to number 60 in rankings by
Rivals in June 2022. In his senior season, he rose in rankings from each service with On3 ranking James the 9th-best player in his class . On January 17, 2023, the
Los Angeles Times reported that James would decide between attending college at
Ohio State,
Oregon or
USC. On May 6, James announced his commitment to USC, with Ohio State being his other finalist. ==College career==