A student is excused — is that session lost or can it be made up?

Not lost. Attendance has three marks: Present deducts one session from the pack, Absent burns one, and Excused deducts nothing — it grants a make-up right instead, with a deadline your school sets.

A make-up right is booked into a suitable session of another class. If the student attends the make-up, the right is used; if they are absent, it is burned; if they are excused again, the booking is canceled and the right returns to available as long as its deadline has not passed.