It is possible this policy does not adhere to the guidelines on managing non-attendance set out by the Victoria State Government's Department of Education and Training. A total of 13 missed classes in a subject equates to less than a tenth of the typical year, and in the absence of exception for greater medical absences, may also go against the Victorian Curriculum and Assessment Authority's Special Provision policy. Given the way that VCE units are structured, an automatic non-satisfactory grade for 13 periods of absence is likely to impact certain demographics much more than others, and that school administrators are sure to know this, it may be in violation of anti-discrimination laws as well.
This issue should immediately be raised with the pastoral support staff (pastoral or "home room" teacher, deans, guidance or health support), and with the school council student representative (if there is one), and with the student's parents or caregivers. If it does not already, the school policy should be clear on the manner in which it complies with the broader policy and guidelines of the DET, VCAA, and any regulations that further apply.