"You did it again!" shouts school Athletic Director Val Tintiangco-Cubales, halting play with a shriek of his whistle.
He instructs a player how to throw a more strategic inbound pass.
"You turned your back, that was your demise. Get the ball and open up, face the court. OK, hustle up, do it again, let's GO, let's GO, let's GO!"
Breaks are few and far between in the two-hour practice, as players run for lay-ups, do baseline sprints, and practice free throws and three-pointers. Even the water breaks are timed - 30 seconds to chug down sweet water from green Gatorade bottles.
"He's tough, but we like that," said point guard Di'jon Jones, 17. "He wants everything right, and we give it because he's always here for us."
Cubales, 40, wants it right because as he sees it, he's not teaching a sport, he's developing character - turning his charges into young people who know to take their hats off indoors, relinquish their Muni seats to seniors, and win without gloating or lose without pouting.
"I'm using sport to teach life," said Cubales, who acknowledges that he's known as a "yelling coach."
"I want them to be competitive because they will need to be in college and in their careers, yet they need to know how to handle the pitfalls, too."
Cubales, who also coaches girls volleyball and teaches P.E., says he has a soft spot for his athletes, sharing a similar background others euphemistically refer to as "low-income," or "at-risk."
Cubales grew up surfing and playing basketball in Santa Cruz, and attended a high school with an athletic losing streak that reminded him of Balboa when he arrived in 1997.
Many of his players have parents who work several jobs and can't attend their games. Some live in frenetic homes, without clear supervision.