For more than 100 years, New York’s Professional Children’s School has provided an education for young people already engaged in careers in the arts or athletics. Their alumni include ballet star Tiler Peck, cellist Yo-Yo Ma, and actors Rita Moreno, Scarlett Johannson, and Macaulay and Kieran Culkin. Correspondent Mo Rocca talks to Peck; with Head of School James Dawson; and with some current students who are splitting time between schoolwork and the stage.