"But I knew it would."
One boy who tried every ploy he could think of to avoid writing, is writing essays of 1200 words now, Mr Johnston says.
"I was really concerned for this boy ... but working on an iPad every day has changed his whole approach and understanding."
One of his stories about a boy street racer, presented in booklet style, was read by his classmates who have written comments in the back, such as "Nice bro", "Cool story bro" and "Awesome story with a great ending".
"Now this boy is asking to do writing, he is asking if he can write an essay," Mr Johnston says.
The children grouped around the desks each with an iPad are totally absorbed in what they're doing.
Their chatter is quiet and focused. It is a happy classroom, cohesive in its learning.
Mr Johnston is like the master of a ship whose keen crew is sailing with huge confidence in their leader.
At one table, Mathletics is under way. Mathletics has been a great addition to the collection of resources students can access, which enriches their learning experience, he says. It is a unique product that tackles a number of challenges in teaching and learning mathematics.
Students love the competitive nature of Mathletics, Mr Johnston says.
"They have fun and they're learning at the same time. It even motivates them to do mathematics homework. My students really love it ... isn't that great, they're developing a real love of mathematics ... you just can't get better than that."
A great roar suddenly went up from the maths desk, one girl was so excited she jumped up, clapping her hands.
"You see what I mean ... they really get excited and it's great to see."
In previous years some Year 7 and 8 students would leave the small Catholic School and head off to an intermediate school, Mr Johnston says.
"I think their parents thought there would be more benefits in going to a bigger school with more students and more choices. But the reality is they are mostly better off staying here, with a smaller class, and being fully digital."
So far this year at St Marcellin the number of year 7 and 8 students leaving to go to an intermediate school has reduced, he says.
"Well, parents are seeing that their children are very happy here and doing so well."
However, Mr Johnston cautions that using the internet for learning can be a double-edged sword.
"Children's learning doesn't suddenly improve because they are using the internet. They need to learn how to use it properly, not just use it for games, any kid can do that and most do. These kids are learning to use it properly, which is why their learning and achievement is rising dramatically."
A group of students at a corner desk are busy with their CVs.
"They're writing their CVs with a letter applying for the positions of head boy and head girl, giving their reasons why they should be selected for the position."
Science and social research booklets are pinned across the walls, showing ample evidence that these students are well-versed in research and finding all the salient facts for every project they do.
Again Mr Johnston points out one detailed booklet about the rocky shore and some of the special creatures living there. The booklet is bright, interesting and full of information.
"Now, again, this is by a boy who hated writing, really hated it ... it's good, isn't it?"
On another wall are projects the students have done about the 1960s and things from the 60s that have had a lasting effect on life today.
There is everything from fast food to comic characters, such as Batman, and the space race.
A series of headlines catches my eye and Mr Johnston laughs as I read "Disney is a fake", "Disney is a fraud" and "Disney is not real".
The students' criticisms have come from watching the Disney animated film of Pocahontas.
"After they were watching the Disney version of Pocahontas they researched the real facts about Pocahontas and found the stories quiet different and wrote about it giving their opinion of what Disney had done with the real story in his movie.
Mr Johnston says there is no doubt the iPads have worked educational magic.
"Because it is so much easier for the kids to edit their work, so the quality of their writing is just escalating.
"These kids have also become so much more productive and it happened in just weeks."
They now have access to extensive research at their fingertips which means they're writing more easily because the smaller keyboard is more suited to the size of their hands, he says.
"We're like a learning bubble in here."
And as well as stories around themes such as bullying and spreading rumours, all written as creative and entertaining pieces, the students are also making movies with a special programme.
The theme is: What are the good things about Wanganui.
They're out there finding all the positive things they like about their town, Mr Johnston says.
"It's good for them to do this and they really think about it ... they have come up with some great stuff."
The success of his students has delighted Mr Johnston.
"Well, they're just doing great, they really are," he says.