The student is not being penalized for absences, as a matter of fact, attendance can not be used to solely reduce a grade of fail a student. @jrm815 pretty much summed up what is happening. The student absent is results in the student not being in class for participation purposes, if they have a "do now", in class pop quiz, group work, classwork, in-class assignment or some sort of exit ticket, done at the end of the lesson and he is not there to be graded for that assignment.
@sybbie719 Teacher is perfectly within his her right to set up their syllabus however they choose as long as they are establishing a policy that is being consistently applied to all students in the class
No specific policy regarding missed classes due to school sanctioned activities or due to taking IB and AP exams, i.e., missed classes are missed classes no matter the reason.
It seems like a poorly designed IB program that penalizes kids who miss an IB class while they are taking the IB/AP exam for another IB/AP course.