Student or Learner
If I want to say that I cannot go somewhere and attend something because I have to be at school, or do homework, or do something related to my school, can I say that I cannot go to that place because of my school duties or is there some better (common) phrase that describes all your school obligations and tasks (attendance, homework...)?
If the work is in a class or classes, just say you have school.
If the work is outside school (like homework), just say you have school work.
I'm not a teacher. I speak American English. I've tutored writing at the University of Southern Maine and have done a good deal of copy editing and writing, occasionally for publication.