Jalen Green, one of the best high-school basketball players in the country, always planned to reveal his post-high school plans on social media in dramatic fashion. It wasn’t until recently, however, that his plans didn’t involve at least one year of college.

Green, the No. 3 player in the 2020 class from Merced, Calif., announced via Instagram on Thursday that he would skip college and sign a contract that pays him more than $500,000 to play for a developmental team sponsored by the NBA in California. That makes him the first...