Can they force me??

Today I am scheduled to work from 8:45 a.m - 1:00 p.m. So far the girl that is supposed to come to work and work the last half of the day hasn't shown up. She is 15 minutes late. Can my work force me to stay and work the last half of the day even though they didn't schedual me for it? Am I allowed to say no and leave? I have things to do and can't stay.

I am a cashier.