Mr Nelson said: "Hitting a hundred is really cool. It means you don't have such wide age groups in the class, and most classes are that much smaller.
"There will still be 30 senior students but the other classes across the school will be smaller."
The school, which was built in 1989, had a capped roll of 130 pupils and also stood to expand to six classes in 2016 if a healthy spread of senior pupils stayed on for that year, he said.
"There's lots more kids coming in next year - we've got 22 already for the new entrants class - and there will be 18 Year 6 pupils next year.
"So we hope those older kids will stay on the year after as well, and we could, in theory, go to a sixth class."
Mr Nelson's daughter, Ally, will be enrolled next year as a Year 7 pupil at the school and he was confident the senior classes would be special.
"I believe our senior classes will be so strong, I want my daughter to be a part of it. There are options for intermediate education in Wairarapa, but we want St Mary's to be the school of choice."
Mr Nelson said the school would focus on pupil achievement and goals surrounding its special character of social justice and "teaching to help others and helping others to help themselves".
A trip to Vanuatu was on the agenda for senior pupils next year that would dovetail with French language classes and the special character of the school.
"We'll take gifts like sporting gear to give out over there, and the following year help a couple of pupils to visit here from the school we visit."
