When I was a part-time sup, which last was about 7-8 months ago, the same rule was in effect. I had no problem with this, because I never wanted to work over 5.5 hours per day. However, when push came to shove and there were no other real options, I worked the hours. I usually made up the time within the week by coming in later or going home earlier.
Some in the management ranks will want you to keep working but keep it off the clock and essentially fudge your own time card. This is the minority though, at least from my experience. If your leaving early and not working overtime affects your operation bad enough, word will eventually get up to the decision maker and overtime will be approved.
Other wise, work till the job is done. And on Friday, call it quits once you reach 27.5 for the week. What does your full-time sup have to say about it? What does he/she want or expect from you?
When I was a pt sup, my management was very approachable and we communicated well regarding every policy, big or small. Hopefully you are in a similar situation. I now know as a driver what it is like to have management who only hear what they want to hear. It can turn a happy work place into a demoralizing one.