Sade Alvarez-Gibson, 15, raps into a studio microphone in a makeshift booth made of blankets, while Oladele Kiambu, 17, records: “Keeping my mind focused / I’m controlling my reality / My grind’s an everyday thing / It’s something that I do casually,” she raps. Just feet away, Kamal Muhammad — wearing headphones and a face full of determination — sits in front of a computer monitor, rhythmically tapping on a keyboard, eliciting sounds of a synth and a drum kick. “This is my first time making a beat,” the 12-year-old said proudly. All three are hoping to break into the...