"It would he highly unlikely that someone tests positive for anything other than a substance they shouldn't be using.
"If, for example, they are taking some medication and they test positive for that medication, they can apply for a retro-active TUE, which means they would have an exemption to take that medication.
"We think we've taken some pretty significant steps that we normally wouldn't take - we don't take the same steps for a teen swimmer or gymnast competing in the open environment."
Tibbutt said the testing programme results from studies into supplement and drug use at school level, and a "Good Clean Sport - Youth" education programme that has helped prepare teenagers for professional sporting careers over the past 18 months.
A 2013-14 survey showed "concerning" attitudes towards the use of supplements in school rugby and some players even admitted to taking performance-enhancing substances.
But Tibbutt said education wasn't enough.
"At the end of the day, we needed a deterrent element to this programme. It's like having drink/driving ads on TV, but not having a booze bus on the side of the road.
"That's why we're taking this step - we've been pushing the education line and now, at this level of competition, we feel it's appropriate to have some testing.
"The scary thing is we know these students are heavily taking supplements and there's a massive risk to health, but these supplements can also be contaminated with prohibited substances."
Tibbutt expects the campaign will spread to other school sporting competitions, but rugby presented the greatest pressure to students already being recruited by professional teams.
"International research shows that pressure, coupled with supplement use, which we know is a massive issue, leads to an increased risk of doping."
The first XV competition is sanctioned by NZ Rugby, so DFSNZ is within its rights to simply turn up unannounced and test under the World Anti-Doping Code.
"But we understand that's not giving people the best chance of educating themselves," said Tibbutt.
Players testing positive could face bans from all sport.