How to talk to managers?

I am a full time college student and I also work part time for a large movie theater chain. Every week, I send in my work availability requests for the following week. I could do a default availability, but with school, my schedule changes a lot. I always make sure they fill the availability requirements and that they’re sent in on time.

For the past three weeks straight, my availability requests have been approved, but then I am scheduled for a shift that is either partially or entirely outside of my availability. I know that legally they are allowed to do this, but their practice in the two years I have worked for the company (at varying locations, but still) is to adhere to approved availability. I want to bring this up to the managers, as I am sure there will soon be a time where I can’t change my entire schedule last minute to accommodate an out-of-availability shift, but I am also aware that I’m a dispensable employee (by nature of the job, not my performance!). Is there a way to politely ask them to schedule me within my availability, or do I just have to suck it up?