My class has 20 students — do I mark them one by one?

No. There is a mark-everyone-present button, after which you only change the students who are excused or absent. One save for the whole class, and every student's balance is deducted from that moment.

Students whose pack is empty or expired are left out of that button and their row is disabled with their state spelled out, so no presence is recorded without a session to deduct it from. The rest saves as usual.