Strathclyde Police crash investigator Sergeant Stuart Bell, 43, said he believed Raymond Munro approached a bridge at excessive speed before crashing in bad weather.
Natasha Paton, 17, from Cleghorn village near Lanark, died after the bus collided with a bridge on the A73 near Biggar and tumbled down an embankment into a river.
Mr Munro, 63, was at the wheel, taking 39 pupils and staff from Lanark Grammar School to Alton Towers theme park in March 2010.
Yesterday, the inquiry at Lanark Sheriff Court heard Mr Bell blame Mr Munro's driving for the bus leaving the road.
Under questioning, he said: "In my opinion the driver has approached the bend at a speed that was excessive for the bend and the prevailing road and weather conditions.
"He has not been able to maintain the route and has continued straight ahead, virtually demolishing the bridge parapet, and the bus in its entirety has fallen from that bridge into the water below.
"If he had been travelling at a slower speed he may have stopped when he struck the bridge.
"For the whole bus to leave the bridge suggests to me there was an error in determining his speed in making that turn."
The inquiry had earlier heard that a tachograph report had shown the bus had been travelling at around 25mph when it struck the bridge.
The inquiry also heard Mr Bell criticise the way Mr Munro had handled the bus in the moments before the accident.
He said: "It is not a good driving technique to brake on bends. You unsettle the vehicle and it can cause it to lose control.
"It could be there is some form of ice or snow on the road and the greater the speed the more likely there would be a loss of control due to that.
"At the location the standard of driving shown was less than I would expect.
"He should have braked more on the approach to the bend and streered smoothly round it.
"The driver has failed to appreciate the road and weather conditions and driven at a speed that put himself and his passengers in a position where a loss of control has occurred that he was unable to recover from."
Mr Bell said that although Mr Munro's driving errors had serious consequences, they were at the minor end of the scale.
The inquiry also heard a transcript from Mr Munro's police interview.
He told officers: "I was wanting to stop but when I applied the brakes it was black ice, which I didn't know.
"I tried to stop the coach, but it wanted to go its own road. I shouted to everybody in the back, 'You will need to brace yourselves because I am going to have to ground this coach to stop it'.
"I couldn't avoid hitting the bridge because it was sliding into it. I thought the best part of the bridge would be the corner bit, which would be the strongest.
"The next thing I remember I am under the water."
Mr Bell said his investigation had not been able to determine if black ice was present on the road. However he said it would not clear the driver of blame if it had been.
He also said there was 'every chance' that Natasha would have survived the crash had she been wearing a seat belt.
He said: "I believe Miss Paton has been thrown from the window in the absence of wearing a seat belt. In my opinion that may have been exacerbated by other passengers not wearing a seat belt.
"This is more likely to have occurred at the impact on the ground rather than the impact of the bridge.
"I have seen collisions a lot faster than this one, of up to 100mph, where people are restrained within their seats."
The inquiry before Sheriff Nikola Stewart continues.
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