Do I Have To Tell My Boss I'm Leaving When My Shift Is Over?

My boss has a bad habit of either asking me to stay late or not letting me leave until another coworker shows up or the line isn't as long. Recently, I've started sneaking out without a word once my scheduled shift is over. My boss reprimanded me for this and demanded I tell her when I'm leaving because it's "respect." This upset me greatly because not only do I suspect she's asking me to do this just so she can force me to stay later but she lets other employees leave without saying anything all the time. I've never seen her reprimand them. Why am I treated differently? Should I go to HR? Is this supposed to be a requirement?

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