Led by instructor Jodie Sullivan and accompanied by teacher Tony McClean, they headed up to the small dam and through a wide body of water towards a blind corner.
There was no sign they were headed for disaster, a water trap that was gathering immense force in the Mangatepopo headwaters 15km away.
The gorge proper lay just ahead of the excited group. Once there, the student's eyes and ears would experience the same as everybody else, that same world-of-its-own feeling.
Little thought would have been given to what was happening outside, let alone at the Mangatepopo's source high on Mt Tongariro.
When the water level started rising about an hour into the trip, the stream began to roar. The water was no longer speeding - it was fuming.
Thoughts would have turned to survival. There was no way of going upstream, no way of getting out the top of the cave-like gorge, and the students going back the way they came meant risking drowning in what had become a swollen river.
There was no way out. They climbed to a ledge in the gorge and prayed. Their only hope was the stream would drop. It had filled up to half of the gorge already. Would it stop?
The water had started as a trickle, 1400m above sea level on Tongariro.
The rain on Tuesday meant the Mangatepopo's source would have been a little higher than usual at about 1800m above sea level.
Given the steepness of the mountain that close to the summit, it would add little more than 300m to the length of the stream.
The extra water here would be making little difference to the stream's size anyway, being sucked into the porous ground.
Still, it was a start as it headed down into the Mangatepopo's triangle-shaped catchment area that takes up some of the view enjoyed by trampers doing the iconic Tongariro Crossing.
In the 10km down to the bushline, water would have flooded in first from the summit behind.
The next catchment is a hillside several kilometres long that slopes in from the left. Then there is one that slopes in from its right. All the time other creeks that were filling fast were feeding into it, easily 50 while still on the mountainside.
This, of course, was not unusual. It is the way the stream has always worked. The rain, while heavy, was not out of control.
But with only a handful of rainy days since summer began in December, it was the dry land that was different, says Tongariro river expert Tyronne "Bubs" Smith.
The Department of Conservation ranger and local Maori leader grabbed two kitchen sponges to explain the flood to the Weekend Herald.
He wet one, then dropped water on it - it was soaked up.
The other was left dry and hard, and when the water hit it, ran straight off and on to the kitchen bench at his Turangi villa.
"The catchment was just so dry on Tuesday," says Smith, "that water was hitting the surface and running until it found a creek, a river, or whatever it could get into."
Smith describes the hundreds of feeder creeks that join the Mangatepopo before it hits the gorge as like "the reverse of capillaries to a vein".
As the Mangatepopo ran into the bush, it was joined by more, often bigger, tributary creeks, all filling in the same way. Its blood was really pumping.
The Elim Christian College students wouldn't have been aware of any of this.
The gorge was filling with water. Having come in from the bottom, they also wouldn't have known what was happening upstream around the next blind corner.
River levels in the gorge were fine when they went in. They had passed signs warning how quickly it could rise as they walked to their entry point at the bottom. They'd be making their way up, and could turn around and go back down if there were any problems.
They took shelter on a ledge in the gorge. Would it drop again?
The instructor Jodie Sullivan would have heard how quickly it could rise from other staff, but having worked at the centre for only three months is unlikely to have ever seen it in rain.
An outdoors junkie, she would be all too aware of how fast rivers can rise - her Facebook photo is of a river scene.
The decision for the group to go in was made by the centre's field manager with 10 years of local knowledge.
The stream explodes in the gorge several times a year but usually with much more rainfall.
The daily MetService weather report faxed to the centre that morning had forecast rain. Questions remain about why the centre did not get a later report that forecast flooding.
Even so, there was nothing to measure the speed with which the water was pouring off the dry land.
The water in the gorge would not have been like a tidal wave. It would have been steadily rising, the water filling the gorge as it would the hallway of a home.
The science of what was happening was that the stream was running at 0.5 cubic metres a second at 3pm, but within half an hour had reached 18 cubic metres per second at 3.30pm - the equivalent of the Tongariro's flow.
The roar - part of the gorge's attraction - was increasing too, just like that of a jet engine. It would have been difficult for the students to hear each other.
The power of the water ahead of them meant there was no way to head upstream. The cave-like structure of the gorge stopped any escape out the top.
Going back to where they came meant they could get taken with the river and thrown over the dam.
So they waited. They prayed. They sang songs and geed each other up.
Eventually they would be swept off the ledge. They had to leave it before they did.
The reasons for not going downstream had not changed. Once out of the gorge they would be back in the more open part of water that still had steep cliffs on either side.
If they couldn't get to the sides, they would be thrown over the 7m dam used to collect water to generate electricity for Genesis Energy.
And that is exactly what happened to the first of the students to go, 16-year-old Kish Proctor.
The plan was for Sullivan to help catch each student as they came. Kish sped past her and over the dam, thrown over its smooth concrete ramp down into a jumble of rocks and logs the size of cars.
Fellow survivor Sarah Brooks, 15, went over the dam too; they were the only ones of the group that did and survived, the other two students and Sullivan somehow getting out beforehand.
Four of the seven dead were found a short distance from the dam. The fifth body was not much further downstream.
The body of McClean and another student were found 3km down river, another sign of the stream's powerful burst.
At normal levels the stretch they were pushed through is bony with twists and turns and braided with rocks. That they could be pushed so far seems impossible.
They were found tied together, a sign that the teacher McClean had lessened his chances of survival to put the student at ease.
By Wednesday morning the recovered bodies were gone. So were the survivors, bused back to their grieving schoolmates in the Auckland suburb of Howick.
The stream was back to normal.
The Outdoor Pursuits Centre was devastated. There had been Maori blessing on the disused accommodation where the bodies were stored and a rahui or ban placed on the river.
The media had arrived and centre chief executive Grant Davidson, who has a Phd in risk management, cracked with emotion as he described the horror.
The Mangatepopo stream is an integral part of the centre. A sign next to a light switch in the centre implores people to switch it off, saying "the more power we use the more water they take out of the Mangatepopo".
The gorge was first explored by centre founder Graeme Dingle on its first course in 1973.
The centre's official history, titled The Adventure Starts Here, describes how they set off from the bottom but turned back as it narrowed and steepened into waterfalls.
The young maintenance man Dick Keith "scoffed" to hear of it and set off the next day determined to reach the top.
Defeated by the final waterfall, Keith returned and pinned a $5 note to the noticeboard for the first to get there - a prize he eventually took a share of with Dingle.
"For testing teamwork in a challenging and stunning natural environment, the gorge has remained one of OPC's greatest assets," says the book.
The gorge has previously claimed a life, in 1976, when a 14-year-old was swept away from her group. The book tells of the death's "long-lasting, depressing effect" and a report found the tragedy could not have been averted.
The gorge was eventually reopened, although it was agreed that only stronger groups should go in.
The future of the Mangatepopo gorge as a centre activity awaits the reviews of the deaths.
Whatever happens, the water will still flow through it.
