Peter Colquhoun, 28, planned the trip from Lanark Grammar School to Alton Towers theme park in 2010.
The coach carrying 39 pupils, three members of staff and two adult helpers crashed into a bridge and travelled down an embankment, where the front end became submerged in a river, killing 17-year-old schoolgirl Natasha Paton, from Cleghorn village, near Lanark.
The first day of a fatal accident inquiry at Lanark Sheriff Court heard Mr Colquhoun describe the moments before the accident on March 31, 2010, on the A73 near Biggar, Lanarkshire.
He said: "As we were going down the slope I saw the wall of the bridge approaching and I knew something wasn't quite right. I remember the driver trying to turn his wheel to the left quite frantically but it wasn't turning enough. Then we hit the wall of the bridge and it fell on its right- hand side into the river.
"My head was under the water. There was obviously hysteria and people were screaming. I got to the surface and I was able to stand and look around to see what had happened."
Mr Colquhoun told how he and a colleague smashed open the front window of the coach in a bid to rescue trapped pupils and fellow staff. He added: "At this point the driver was conscious but he said he couldn't move. He said he thought he had broken his leg and we helped him out of the water and on to the banking.
"My concern was getting the pupils out of the bus and on to the banking. Some of the lads at the back had got the rear skyhatch open and were getting people out that way. When I was down at the front I was trying to feel around under the water as best I could but it was so cold I couldn't tell what things were."
Under questioning from depute fiscal Carrie MacFarlane, Mr Colquhoun told how passengers became separated on different sides of the river before taking shelter in vehicles that had stopped at the scene. He said he only became aware a pupil was missing and had died when he was discharged from hospital later that day.
Mr Colquhoun added: "I was trying to do a headcount to figure out if we had everyone but it was really hard as I didn't know where everyone was. Everything was so chaotic and hysterical and everyone was in shock."
The court also heard Mr Colquhoun, who suffered a cut arm and a head wound, raise doubts over driver Raymond Munro's speed. He said: "The bus went too fast to negotiate the corner. That was because of the way he was driving it. If you were crawling along very slowly you would be able to take that corner. But I don't know if it is for me to say if he was driving too fast, I am not an expert."
Mr Colquhoun said there had been no discussions about cancelling the trip.
He said: "It was snowing the night before, but snow happens in Scotland and people still go on journeys, and I checked with the bus company and I was happy with that."
Mr Colquhoun said a pupil had Googled their destination and given it to Mr Munro to enter into his satnav.
The inquiry, led by Sheriff Nikola Stewart, continues.