What is per diem??

I’m in school as well as my first job in my profession. I told them I can only do per diem. My classes are incredibly stressful and one assignment takes about nine hours to complete. I also have tons of group work. I’m in orientation at my job, and I work every Tuesday. I want to tell them I can work every thursday as well, especially how it’ll be good for orientation purposes my days are close by and my patients will know me. They have me on every other weekend. This month i’m scheduled for three weekends (saturday and sunday, both about ten hour shifts). Am I required to work weekends if i’m per diem? I understand if ur full time or part time, but from my understanding I thought per diem is when you work the hours you can? I’m stressed out and having massive panic attacks. i’m going to get so sick. I can’t do this. If I have my weekends free, I can do my school work during that time and my group work, and this way I can give my job work weekday shifts and it’ll help me with orientation purposes. do I have a right to ask for that? I honestly feel like they are using me and pulled a fast one on me because i’m new to the work world and don’t really know what i’m signing up for. I feel so stupid and upset. I cant stop crying.