If a student needs deleting, you need to take the following steps to remove their attendance marks
1) Browse to student profile > Attendance > Reports
2) Click audit history & press the green button to show the audit history
3) Remove the marks from all sessions
4) Go into the session and reopen the register. The mark will be removed.
This video will walk you through the steps.
NB you should only ever delete a student who was NEVER at anytime enrolled in school eg they didn't arrive as they took up an offer of a place elsewhere.