Is this appropriate?

Just some background. Im 17 years old, and have been working at a vet clinic for 2 1/2 years. It’s my senior and instead of asking off all the time during the week I asked if I could just work weekends. I realize I only work weekends, however, I still had some family events that I got approved of in advance by my boss to ask off for. Today my boss texted me saying I was fired. No explanation at all. It was a text message. Keep in mind I have a very good relationship with everyone at the clinic including my boss. Im in FFA and this was a competition for me, one they knew about and were as invested in as i am. My boss knew if I would still be employed as of December 31st I would still be able to compete. Is the way he fired me appropriate?