Lower Manhattan was devastated by the attacks
Mr Buchanan, originally from Aberdeen, had been taking a class of 14-year-olds through a World Cultures course at Randolph High School last Tuesday.
Then the vice-principal broke the news of the attacks on New York and Washington.
He said: "The vice-principal came in. His face was ashen. He said there had been an attack, but he didn't give any details. He didn't want to alarm the students.
"We knew very little at that point. This is the third period. We start at 7.50am."
Mr Buchanan said that in a school with a population of 1,600 it was impossible to keep rumours about the attacks from circulating.
So, televisions were set up in the canteen area to allow the true story to emerge.
Mr Buchanan said: "One of the hard things was when we put on the televisions in one or two select classrooms.
"I saw a boy in front of me realise his father was in there. We tried to get those individuals out. We got them cellphones, which they are not allowed to bring in to school."
The task of dealing with the traumatic aftermath of the atrocity has been difficult, admitted Mr Buchanan.
"Life is trying to get on as normal but there is almost this eerie understanding that although we are bogged down in the minutiae of daily life something very big and lasting has happened.
"We are still burying the dead. We have not let the dust settle to decide exactly what we are going to do next.
"It has been the same all week. It has been a measure of trying to get on as normal and yet knowing that something very strange has happened. Something that has changed our area very dramatically."
Phoned his family
Thousands of Scottish people live and work in New York and so far four Scots have been confirmed dead or reported missing in the tragedy.
Scottish Justice Minister Jim Wallace has warned that the total number of Scots killed in the atrocity may rise in the weeks ahead.
Mr Wallace said that figure may change as more bodies are identified.
So far, computer consultant Gavin Cushnie from the Isle of Lewis remains among those missing in New York.
The 48-year-old St Andrews University graduate was on the 104th floor of the first tower to be hit.
Banker Derek Sword from Dundee has not been heard of since he phoned his family from the 89th floor of the second tower to be hit.