Mark a whole class in under a minute. Everyone starts as Present, so you only tap the absentees. Then let the parents of absent students know.
Click Attendance Entry in the left sidebar, or Mark Attendance on the Dashboard. Teachers can also mark attendance from Teacher Mobile.

After you submit, you'll see Attendance Saved! with the totals, e.g. 3 Present • 0 Late • 1 Absent.
Parents of absent students who have an email address are emailed straight away. Each absent student is listed with what happened, for example Link ready. Tap Send. Email sent. With WhatsApp connected, the WhatsApp message also goes out on its own. Until then:
The wording comes from the Absence Alert template in Communications › Message Templates. Every message is recorded in the Message Log.

Choose the same date, class and section again. The statuses you saved load back in. Change what you need and tap Submit Attendance. Your changes replace the earlier marks; each student keeps one record per day.
Open the student from the Student Directory and click the Attendance tab for a colour-coded calendar and the year-to-date percentage. Parents see the same in the Parent App.