Who should I go to? Should I call Corporate?

Before the summer began I switched positions at my job, therefore I got a new manager. I gave her my avalibility for the summer which was open everyday but Saturday. She asked me if I could do Saturdays and we agreed to every other Saturday. I told her in the Fall I start school and my schedule would change based on my classes. She said she did not care because she was going to be on marenity leave. So I had to deal with my hr representive, who I bud head with. I changed my hours and there was an issue and she told me I had to work Saturdays. We had a huge agrument. She took out termination papers and told me if I didn't like it here then fill these out. I asked it I could have my old position back as a cashier where I would be given what I needed and she told me that because I didnt sell credit cards I was on the verge of being fired but since I switched I saved myself. The next day I came in and told her I would work Monday, Thursday, and Saturdays approx. 15 hours a week. She approved it. Well now my manager is back. She is putting me whenever she feels like it basically and I am a full time student. I go to school four days and I see a lot of doctors for medical reasons and Tuesdays are my appointment days. This avaibilty ends 12/23. She told another empolyee that she cannot wait to see me Saturday because I will not be smiling when I see my hours. We have 14 people in our department. I feel that if my hours were approved why are they now taking them away. My manager and my HR rep are close friends outside the work place and I feel neither is willing to work with me. Should I call corporate I mean I do not want to cause a huge riff b/w them and me but I dont know what else to do???
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Additional Details
They have also not been approving my days off. I requested two hours off and it was denied. And I requested Christmas Eve because I will be out of state and they denied it.

If you are dealing with pals and such in various departments who are buddy-buddy, your case simply doesn't matter unless it's a union job. Those 'pals' WILL find a way to get you gone. I feel that they do want you to quit - and hire some suck-up that will do a much poorer job than the quality work you do for them. Time to look for a better employer - but you have to stick it out until you find one. This is one of the worst managerial cases I have heard in a long time. They want you out because you are good at what you do - hence, you are a threat to their positions and you must be eliminated now.

I could go on and on - stick it out and find another job. This company obviously has too much of a 'grapevine-gossip' thing going on between so-called 'managers'. You deserve better.

I actually was in a similar position. I quit. I got a new job. Screw you. They called me back, and said please, please come back. So I did. On MY terms. And then they started their old BS again. So I quit again, never to return. The company is doing poorly now. They managed to disrespect all the good workers and managers. And we all went to work elsewhere.