How do i call my boss and tell her i can't make it to a shift i'm scheduled for.?

A local tv show is coming to shoot at my work. I told my boss that i wanted to work that day to be on tv. even though it's during my sociology cl. I looked at my syllabus and found out that the same time i'm working is my sociology midterm. How do I call my boss and let her know that I won't be able to make it to work. There is no one that can cover the shift for me.