What this is measuring?

The attempted column is the percentage of students who attempted the NCEA level (in the current or previous years) out of all the students for that year level.

The achieved column is the percentage of students who achieved the NCEA level (in the current or previous years) out of the students who attempted it.

The data is for the 2017 school year, and is available here.

Why are some schools so low?

Some schools participate in alternative qualification frameworks, like the International Baccalaureate or Cambridge International Examination systems. These schools can have a low NCEA participation rate.