A student does not show up without telling anyone — is the session lost?

Yes. Absent burns one session from the pack and grants no make-up right — that is the difference from Excused, which deducts nothing but grants a make-up right with a deadline.

If word arrives late and the school decides to accept it, the admin simply corrects that mark to Excused. The correction repairs the pack consumption and creates the make-up right, with no manual adjustment anywhere else.