Boss is mad I didn't come in on my scheduled

I know this sounds like I'm being a spoiled brat but I made it extremely clear to my boss that whether or not she lets me off I won't be at work that day. I put in this day off request two MONTHS ago. I'm in school to be a teacher and it was REQUIRED I go to this school to get my field hours in with special education. I let my boss know the importance of this and she said ok. Then she decided after I kept reminding her to deny it. I asked her why and she couldn't give me a reason and said a day off request is just request and she isn't obligated to honor it. I get that but this is important. Me and her argued about it and I told her I won't be here regardless. I can't miss my field hours. She told me I have a job to do. I said "That's right. This is my JOB! My schooling is for my career and I put this in months ago." If I have to choose between failing my class (because not doing your field hours is an automatic fail) and losing my minimum wage job I'll choose the latter. And I didn't show up. I didn't lose my job but when I say this lady has been so petty for the last 3 WEEKS! She has given me the silent. Completely bashed the field I'm going into. Anything I'm around she talks about how teaching is a stupid field and they need a pay cut not a pay raise. She will ask someone else to tell me something right in front of me. I'm not gonna say I handles everything perfect but I JUST turned 21. This lady is in her 50's. It's been 3 weeks! Grow up!