Say, hypothetically, your job likes to jerk you around in ways

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Say, hypothetically, your job likes to jerk you around in ways that are inarguably unfair.
Example: worked for a company for 5 years now, transferred location last year in Oct. I stated I needed a specific shift in order to attend school (main reason transfer was successful) instead of the one I had instead been switched to at the LAST MINUTE. As of last month they gave an individual who worked here on a seasonal basis that shift and the job title I had been told I would be recieving.
Essentially my boss bullshit me saying this was due to vacations being difficult and because it was THE ONLY SHIFT THAT WOULD WORK FOR THIS BRAND NEW EMPLOYEE in a company that does its decisions on a seniority basis.
The union (inb4 alt right fags, I don't care for the union) is worthless and will do nothing to help me, and the HR for the hub I'm in technically doesn't exist because the one lady who works in it is on disability.
TL;DR how do I make shit happen at my job so that I either get my desired shift so I can go the fuck to school, OR how do I move on in a way that will completely fuck over this little hub and the company in general which plays favorites and shits on hard working individuals?