An Education Review Office report indicates 75 percent of secondary schools could do more to help their students get the National Certificate in Educational Achievement (NCEA).
The report says only 10 of the 40 schools it looked at in 2013 are effectively good at figuring out what extra support their students need, providing it, and double-checking that it is working.
It says a third of the 40 schools could do more to ensure attempts to help students are effective, while some teachers at the 16 worst-performing schools lacked a sense of urgency.
The review office says the most successful school tailors what it teaches to the interests of it students and it encourages all schools to do the same.
The report indicates many schools could do more to help meet the Government's goal of 85 percent of 18-year-olds having at least level two of the NCEA by 2017. Currently, the figure is 78.6 percent and annual improvement has slowed.
Stephanie Greaney, the ERO's evaluation services manager, says schools can improve their NCEA results by tailoring lessons to students' needs and interests.
"What this report has identified is that the mentoring, which has been really improved for individual students, can only do so much. And in the end, curriculum changes need to be made to make sure that the students are actually doing the courses and programmes that they want."
Ms Greaney said possible changes include teaching maths as part of other subjects, such as food technology or biology.