AITAH for leaving work early?

So I (22F) work part-time at a private school. I am the choir accompanist for the high school choir, which meets on Wednesdays and Fridays from 2:30-3:25 pm. I work alongside the high school music director, "Bob" (51M), and usually the middle school director "Marge" (45F) joins as well to help with rehearsal. Today was Friday, and I was running a little late to work, and I was cranky because I'd skipped breakfast. I pulled into the parking lot right at 2:30 and dashed up the stairs to the choir room, only to find that Bob and Marge were both absent and a substitute was working in their place. The poor substitute had no idea what to do. She had apparently been sent in last minute with no preparation as to what this class was expecting from her. (She probably didn't even know how to sing or read music, either.) I checked my phone for any notifications from Bob or Marge that I'd missed, but there was nothing. I'm guessing they were both ill. But the fact that neither of them thought to inform me of this pissed me off even more. The students understood that they probably wouldn't be able to rehearse without the two main teachers there, as I wouldn't be able to play the piano and conduct the choir at the same time. A few of the choir students asked to borrow the piano so they could rehearse on their own. The other students pulled out their school computers to finish up some homework, and the substitute basically just took attendance and acted as a chaperone. I had no purpose there anymore, so I left. It was 2:52 when I left. I went to get food (at last) and then I put gas in my car and then came home. On the way home, I couldn't help but tell myself that what I did was selfish and unprofessional. But then I told myself that Bob and Marge not showing up to work without so much as a text saying why not was equally unprofessional. Am I an A-Hole?