ATLANTA — Asha Crews had never played a guitar before this week. But on Wednesday, she took the stage in her favorite Michael Jackson shirt, strummed a few chords and shouted, “One, two, three, four!”

Then her band, Rainbow Unicorns, began to rock, playing an angst-ridden song that sounded like early ’90s grunge. “It’s all complicated, I feel so frustrated,” sang the four girls, ages 10 to 13.

This was not a standard garage band. For one, the members had picked out instruments two days earlier. For another, the jam session ended when a supervisor called, “Ladies, it’s snack time.”