Anyone opening the door would be met by a fug of cigarette smoke, with puffing teachers barely visible through the nicotine haze.
But all that changed with the 2007 ban, and teachers now have to find other places to light up.
As the UK marked No Smoking Day on Wednesday, TES Cymru found that most schools in Wales have adopted a zero-tolerance policy to smoking anywhere on site.
And heads say this tough stance has had the added benefit of reducing the number of pupils caught smoking.
Bryn Hafren Comprehensive in Barry once had a separate smoking room, popular with several teachers.
But Phil Whitcombe, its head, said staff were warned far in advance of the ban that smoking would no longer be allowed on site. "Surprisingly, it wasn't an issue," he said. "We had no problems whatsoever."
St Cyres School in Penarth is also completely smoke-free. Head Brian Lightman said: "We want to give a consistent message not to condone smoking at all. Any staff members who feel the need to smoke have to go off site."
Mike Pickard, head of Blackwood Comprehensive near Caerphilly, introduced a school-wide ban several months before the national policy came into effect.
He saw this as an important way of stopping pupils smoking. "Seeing adults in a school smoking gives you a very poor starting point for discouraging smoking among pupils. Teachers should be seen to set an example," he said.
Since then, there has been a marked drop in the number of pupils caught smoking in school.
But there is a hardcore of teenagers at Blackwood who still smoke. Surprisingly, they have turned to their teachers for help.
The school recently launched a weekly session to get pupils to kick the habit. The 10-week Quit scheme helps pupils understand the causes of smoking and looks at why they took it up in the first place.
So far 10 pupils aged 13 and above have attended, many voluntarily.
Mr Pickard said: "I was really surprised to have children of that age come to me and say, `I just can't quit, I need help.' It's early days, but we hope the course will have a positive effect on those pupils and others who want to stop smoking."