They probably locked her in as a joke by tying the doorknob to something or putting a chair against it, and after a short while somebody let her out or she got out on her own. Just typical teenage horsing around.
Teenage boys should now ask girls if they want to be “Feinsteined” and then put them in a room. :-)
Interior doors don't open out, they open inward. A chair against a door only works from the inside of the room. The direction the door opens. The doorknob could have been tied up though.