A common complaint from examination teachers, especially those teaching IB: students miss many of their classes due to college visits, community service, fieldwork for other subjects, music or sports trips and so on. However, these students are "present" because these are official school activities. One colleague who keeps his own attendance records tells me he has students who missed more than 60% of his classes because of such activities. Their report cards show perfect attendance.
Teachers know the problems of missed classtime, even with highly-motivated, high-achieving students. Material must be repeated, and lessons re-taught. Intermediate or advanced concepts or tasks must be delayed because essential preparatory concepts or tasks were missed. Assessment deadlines typically cannot be moved, so parts of the program are missed or students enter the assessment poorly prepared.
A related problem exists with the high-stakes role of attendance figures. US charter schools are rewarded for high attendance so the incentive exists to inflate numbers. UK league tables rank schools based on attendance providing the same type of incentive. What gets rewarded gets done.
I read a UK report where a student was permitted to miss every Wednesday because of his family's business, yet his report card showed a 94% attendance.
What can be done? Obviously the community service, student council, sports trips have value, in fact are probably essential. Equally obviously, attending class has value, in fact is probably essential. The superficial answer is one of balance. Yet every activity has its booster arguing its importance over others and which school leader wants to be in the position of determining relative worth.
I worked in two schools with exactly this problem but two different approaches. Both were IB schools with long experience of the program. One allowed all such interruptions, but did not make them available to all students. The "better" academic students were rewarded for their genetic superiority by being allowed to run for student council, join the choir, take lead roles in the major production, play varsity sport. (I should add this school also had all kinds of problems, most from the students in classes below the honors groups.)
The other had a specially-designed calendar with designated weeks for trips, a shorter summer and more intersessional weeks during the semester for trips. Student council and so on occurred outside the school day. Long or final preconcert or major production rehearsals occurred at weekends. This school actively monitored student attendance per class. Students who went on a college tour were told absences from specific class would be counted and so they must ensure they met the 85% minimum attendance requirement.
I do not know the answer to this prickly problem. Each school must have its own debate because it is an integral aspect of each school's culture, aka "the way we do things around here". Ignoring it, and/or simply promoting students, and/or simply entering students for assessments where they do not do as well as they should is not one.