Two pupils, Bella Berry (17) and another year 12 pupil, were diagnosed with the disease within a week of each other.
Today all year 11, 12 and 13 pupils at the school would be offered a course of preventive antibiotics to kill the bacteria which caused the disease.
The bacteria was present in the nose and throat of 10% to 20% of the population and usually passed from person to person without causing harm.
Public Health South in consultation with the Ministry of Health and the Institute of Environmental Science and Research, would provide and administer the antibiotics.
Last night, about 120 parents attended a meeting at the school where principal Wayne Bosley and medical officer Dr Keith Reid answered questions about the disease and its spread.
One parent asked Dr Reid why the entire school was not being offered antibiotics.
Younger pupils would have to take a different antibiotic and there was no evidence it would stop the spread of the disease, Dr Reid said.
In a statement yesterday, Bella's parents Graham Berry and Carin Andresen, said her condition had improved and she was expected to make a full recovery.
The other pupil was still in Christchurch Hospital in a stable but critical condition.
Dr Reid said it was almost certain the pupils did not catch the disease from each other or even from the same person, which was why all senior pupils would be treated.
The risk to other pupils, who had not already been contacted by public health officials, was no greater than the risk to the general public, he said.
In his 16 years as a public health official in both New Zealand and the United Kingdom, Dr Reid could not recall two cases of meningococcal disease at the same school in such a short space of time.
Tests confirmed Bella had contracted strain B of the disease and it would be known today what strain the other pupil had.
If it was also strain B, it would be the seventh case in the southern area this year, well up on the number of cases last year, Dr Reid said.
It was difficult to determine why there had been an increase as the other cases were spread across the southern region, he said,
The SDHB would continue to investigate the reasons for the increase, Dr Reid said.
