Why am I being reprimanded for coming to work early?

Tweet

So, I'm a lab assistant for the reading lab at my University and it is required for people to complete so many labs by a cut off date. The cut off date is less than a week away so I decided to come to work early and open up the lab since no one is scheduled to work at that time. I've been doing that for a couple days when one night i came in eight minutes late and the night supervisor chewed me out about coming late and having people waiting for me. So before she could start a whole lecture, I told her that I'm actually an hour early. BIG MISTAKE. So now she chews me out for coming to work an hour early because I have a set amount of work hours (12hours) and I cannot be working outside of my scheduled hours. i don't know why it is wrong for me to be chewed out for working extra hours. I'm not even getting paid for them. I know that. The secretary that calculates my hours told me if I go over 12 hours a week I won't be paid for it. I told her I'm fine with this. I have the spare time. But I got chewed out for it by the night supervisor. Even my boss is shocked at that since she asks me to come in sometimes out of my scheduled hours and help. I told that to the night supervisor and she said that I would have to get a written note from both my boss and the secretary to do this. I don't want to do that since sometimes it's just random. Ok. I know that I did some long ranting there but I needed to get it out. i just don't see why I should be penalized for trying to help out more. What do you guys think?