A class teacher marks the register from a phone while standing in front of the class, and absent parents hear about it before assembly ends. That is the whole design goal; everything else follows from it.
Daily or period-wise
A primary school marks once in the morning. A senior school marks each period, because a student present at nine is not necessarily present at two. Both are supported, and a remark can be recorded against a student when the reason matters.
The whole class in one action
The register is submitted for a class and a date together rather than student by student. Most of a class is present, so the work is marking the exceptions, not confirming the obvious.
Parents told the same morning
Absentees reach parents by SMS and WhatsApp on the number the school already holds. No app to install, and no phone call from the office at four o'clock about a child who has been missing since nine.