Courtney Vandiver walked toward the tall, off-white credenza in the corner of the studio, above which hung two loose coloring sheets with pink crayon streaks bleeding from the bold, black outlines of butterfly wings. About eight feet of blue masking tape formed an “L” shape around the credenza. Before the class began and the girls were still giggling and adjusting their leotards, Courtney told me she had placed the tape there one stressful day, warning her students not to cross the line.