Hundreds of students at Archbishop Riordan High School staged a walkout Friday during an all-school assembly that featured a prominent antiabortion speaker. About five minutes into the presentation, during which staff members patrolled the doors, students at the Catholic high school exited the theater and walked into the gym, since school rules don’t permit them to leave campus. A few dozen of the school’s more than 800 students remained in the theater for the entire presentation. The speaker, Megan Almon, works with the Life Training Institute, a program that focuses on teaching people who are opposed to abortion to “persuasively...