Was it fair that my teacher marked me absent?

Today was the second to last day of school and I was in 5th period orchestra. I had to go to the bathroom really bad (my bladder was seriously about to pop) by the end of class, but my teacher was in her private office testing students for orchestra next year and I didn't want to interupt so I ran to the bathroom. By the time I was done the bell had rung so I left. I was only gone 3, 4 minutes tops. Anyways, in that short amount of time I was gone, for the first time all year she decided to leave her office and take roll at the END of class. I was marked absent and the school called my mom. I quickly emailed my teacher explaining to her that I didn't ditch. I am a good student and I have an A+ in her class, but she wouldn't budge and told me that she would not clear the absence. If I don't get the absence cleared, I will have to serve an after school detention at the beginning of my senior year. I'm afraid that I won't get accepted into the college I want to go to if they see that I got a detention. Do you think the teacher was being fair? Is there any way that my mom can call the office and clear the absence for me? Has anything like this ever happened to you? Thanks for taking the time to read this. All answers are appreciated :)