Pass rates for the NCEA qualification continued to rise in 2011, but children from poor backgrounds and Pacific Island and Maori students still lag, according to new figures.
The Qualifications Authority figures released on Monday also show that schools in Canterbury had the highest pass rates, while the West Coast had the worst.
Pass rates increased at all three levels of the National Certificate of Educational Achievement, reaching 82% at level 2 - the level now considered to be essential for young people's future success.
In Canterbury, the figure was 87% - the highest in New Zealand - while the lowest was on the West Coast at 76%.
Girls' achievement rates were higher than boys' at all levels of NCEA.
The worst results were for Pasifika boys - at year 12, only 60% achieved level 2 compared with 91% of New Zealand European girls.
Secondary Principals' Association president Patrick Walsh says annual improvements in pass rates are likely to plateau.
The Qualifications Authority show last year's NCEA pass rates were as much as 2.3% higher than in 2010.
But Mr Walsh says it is difficult to sustain such improvement year after year.
"One would expect at some stage for those results to plateau and for the incremental improvements to get smaller and smaller.
"I think probably what schools will start to do is to target those groups that are clearly under-achieving - and we're talking about Maori, Pasifika and students with special needs."