Missed a shift due to college, didn't realise till later on

So today I ended up missing a shift at work because I thought I was off work today. The schedule for this week had me off work today but the schedule was taken down last night and we didn't have a new one up in time for me to check. I had my phone on silent so I could focus in class and it turns out my manager called me to check what was going on.

I've told my place of work in the past that I can't work on certain days due to college but they still give me shifts on college days. I value college over my job since its my education but also don't want to get in trouble for missing a shift. What should I do now?

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