Can a teacher make it mandatory for an after school function where there is no transportation?

My son is in a choir class. He was suppose to go to a choir recital at 6pm (school lets out at 2:30). There is not school provided transportation to and from this function. I had to take my father to the hospital and my son did not make the function and the teacher dropped his grade, because it was a mandatory function. I just don't understand how a teacher can give a kid a grade on something that is held after school and transportation is not provided. He is in 7th grade.