Miscommunication in Work

I have some fairly recent experience with miscommunication at work; this summer I was working at a pool as a lifeguard/swim instructor at a local pool in Clemson (not Fike). It was a pretty easy job and I didn't work very often so I was thinking about keeping the job during the school year as it was an indoor pool. However, through a series of miscommunications, I ended up quitting. First of all, I was only scheduled to work on Tuesday and Thursdays because on the other days I have class from 9-4 and would not want to work following such a long day in class, and on those days I don't has class at all. Anyway, on Wednesday during sorority rush week, I got a text message from a number I didn't know asking if I was coming into work. I responded by first asking who that person was and then saying that I wasn't scheduled to work that day. The person told me that she worked at the front desk (not even my boss) and that someone had written down that I was going to be working on Wednesdays. I told her I couldn't come in because I had a commitment to my sorority and thats why I wasn't supposed to work in the first place. She texted back "so you're not coming in then?" and I told her no that I couldn't. Then, my boss called me and told me I was on the schedule to work on Wednesdays. I told her I wasn't, I was only scheduled on Tuesdays and Thursdays. I was then told that the schedule had been revised and she thought she had e-mailed it to me. I'm not the type of person to just not show up to work and I had never recieved an updated schedule. She apologized for not sending and told me I need to make sure I'm there next Wednesday because I was now on the schedule.
The next Sunday morning, the staff received e-mails saying to come to the pool at 5 for training but if you couldn't, let someone know and you would have to swim laps before or after your shifts every day for the next week. I couldn't make it to the training as I had a meeting for the executive members of my sorority at that time, so I e-mailed my boss to let her know and said I would be fine swimming the laps because I don't mind swimming. She did not respond until the next afternoon when she called me and told me that I would have to be at the pool at 8 that night to make up for the training I missed the day before. I didn't get the voicemail until after class (so like 4:15) and was kind of irritated because she hadn't told us training was mandatory and also I have chapter meetings for my sorority at 8. I decided to call and tell her I quit.
The whole situation could have been avoided if there had been better communication between us. I had never gotten an e-mail telling me I had to work and was never told the training was mandatory. I felt that I explained that I was a full-time student and had other obligations at school that I could not miss, and besides, it wasn't like this was a legitimate job, it was just a way to earn some extra money. I ended up quitting because there was not effective communication between the workers and our boss. We should have been informed of decisions way ahead of time instead of the day of and should have made sure we knew what was going on when. Furthermore, I thought it was very unprofessional to have the person at the front desk contact me instead of herself.