Millions of teenagers in high schools nationwide are using a smartphone app to anonymously share their deepest anxieties, secret crushes, vulgar assessments of their classmates and even violent threats, all without adults being able to look in.The After School app has exploded in popularity this school year and is now on more than 22,300 high school campuses, according to its creators. Because it is designed to be accessible only to teenagers, many parents and administrators have not known anything about it.