Tauranga Boys' College principal Robert Mangan said the nature of the NCEA qualification system meant the 80 credits needed to pass could be gained throughout the year.
"External exams won't be essential for some at the end of the year. Those seeking Merit or Excellence endorsement might choose to sit their exams while others might take a more pragmatic approach and say 'I've done enough'. As an educator, it would be nice if students were aspiring to achieve them all but that's an issue that sits within the qualification."
Mr Mangan said some subjects at his school had prerequisites for students to have gained a certain number of external credits in the previous year.
Russell Gordon, principal of Mount Maunganui College, said some students set goals of gaining their qualifications before the end of the year so they did not have to sit exams.
Mr Gordon said scholarship exams were generally the ones people did not show up for.
Aquinas College principal Ray Scott said his school had small numbers of students who would not show up for various reasons, including that they had already gained their qualifications.
NZQA deputy chief executive Richard Thornton said there was a national average absentee rate of 5 per cent per year but some absences were justifiable.