In any case, if you’re going to pursue this, it would be good to read all the materials that your child was given at the beginning of the course, and to ask for clarification of the grading policy with the thought in mind that maybe your kid misunderstood something or maybe you misunderstood what your kid told you. Because in fact, one of you may have misunderstood. Also, a less aggressive approach, at least initially, might be received better.
But at some point, you will probably find yourself saying something like, “I know you want your students to participate in honors musical ensembles and to take IB tests. How could a student do this without jeopardizing his GPA?”
It would be a shame if a school penalized students for participating in honors musical ensembles. Having the opportunity to play with other students of your own caliber can be a very enjoyable experience for a serious or moderately serious musician. It can help keep the kids interested in instrumental music. But as others pointed out, perhaps your son needed to talk to the teacher in advance about the academic consequences of his absences.