Hunter Greene opens the door in his underwear—and only his underwear. It is 5:45 a.m., but he happily offers me a glass of water and welcomes me into his family's two-story home in Stevenson Ranch, California, with a 360-degree view of the Santa Susanas and, just south of the mountaintops, the city that breeds stars. There are African masks and sculptures all over, arcade versions of Ms. Pac-Man and Centipede upstairs, and a painting of Jackie Robinson in the dining room. On top of the kitchen cabinet, a sign: BE YOURSELF. EVERYONE ELSE IS TAKEN. Justice, the family cat, slinks from room to room before, as if programmed, appearing on the kitchen countertop.

This is where a senior at Notre Dame High, working on two hours of sleep after returning from Coachella, gets ready for school. But this particular high school senior wakes up before the sunrise every day—to eat, to take three dozen swings in his backyard batting cage and get to first period by 7:45.

Hunter and his father, Russell, have established a breakfast program by now: omelette, bacon or sausage (corned beef hash on Wednesdays), strawberries and bananas, plus a power-shake blend of chocolate Muscle Milk and spinach. As Hunter showers, exhausted, Russell flips the egg, alive. Hunter and his friends prefer Travis Scott, Lil Uzi Vert, the new Kendrick, but as the grill sizzles, Russell's playlist—Maxwell, Erykah Badu, Floetry—dominates the morning air.

"Hunt, you almost ready?"

"Yep."

It's a production all right, bringing up a baseball prospect as senior spring gives way to becoming the No. 2 pick by the Cincinnati Reds in the 2017 MLB draft. "I'm glad it's almost over," Russell says in the last week of April, nearly two months before the draft.

But this is no ordinary prospect. This is "Doc Gooden," says one longtime California scout. This is the next Jose Fernandez, says a National League executive. And that's just on the mound. At shortstop, this is "Cal Ripken, Carlos Correa, Alex Rodriguez," the executive declares. This young man is also just 17 years old.