My managers wouldn't let me call in because of this, please read?

I'm a senior in high school, and I work just about 30 hours a week, and this week I was off Tuesday to go spend time with my boyfriend cause I never do. Anyways, I have this senior presentation due Wednesday the 20th, and it's work 70% of my final grade. I called the place I work at (it's a fast food place, not giving out the name) and told them I can't come in because I'm so behind on homework and need time to work on my presentation. They said it wasn't an excuse to not come into work. However, one manager has called in for a hangover, as well as several employees in the past. It just doesn't seem fair I can't call in because my education is more important than work. Please give me your opinion on this. Thanks.