You are guaranteed 27.5 hours per week, not 5.5 hours daily. You will however get paid 5.5 hours for an option day, a vacation day, or a peak holiday where you do not work. This is an important clarification, because it means weeks that contain non-peak holidays will result in you getting paid under 5.5 hours for the holiday if you work more than 22 hours over the other 4 days. I clarify this because essentially all of the PT supervisors who work additional hours over a non-peak holiday containing week end up wondering why their check is short, because in verbal discussions they've had with HR, they get told they're guaranteed 5.5 hours per day (usually something like "you're guaranteed 27.5 hours per week, so 5.5 per day") and holidays are paid... which isn't exactly the case.
If you leave during your shift, you need to clear that with your management team before hand, and also need to code it out in PTRS as an unpaid break. Your management team may want you to work additional time to make up for the hours you're not present, or they may not care. If they don't care, you'll get paid 27.5 hours for the week even if you work less than that.