When it was built it drew comparisons with Big Brother and had pupils pressing their noses against the mirror wondering if they could peer through to the other side.
But three years on, the one-way mirrored observation gallery at Whitley Bay High School in Tyne and Wear is attracting the attention of schools across the country.
Its popularity has grown rapidly, with the school now hosting all 200 trainee teachers from Newcastle University each year for sessions to show them lessons in action.
This week it won the backing of the Specialist Schools and Academies Trust, which gave the school its annual national award for impact and innovation.
The gallery, which is sited between a regular classroom and a science lab, allows people to "sit in" on lessons without interrupting teachers or pupils.
Headteacher Adam Chedburn describes it as "the single most important tool in improving the quality of teaching in the past 10 years" at the school.
"Of course, there was scepticism to begin with," said Mr Chedburn. "People said the unions would never wear it and the head would be in there all the time.
"Students also had to grow accustomed to it. They would get their noses ever-closer to the mirror to see if they could see through.
"Others were busy looking at themselves and brushing their hair, but as with most things the novelty wears off very quickly."
When the gallery was built, using money the school received when it became a specialist college, 20 teachers volunteered to be observed.
But by the end of the first year, 80 per cent had signed up and now all of them use the facility to have their lessons observed and to watch their colleagues.
"It works because we have a culture of complete openness," said Mr Chedburn. "The culture of shared observation is so strong that there is no problem."
All newly qualified teachers starting at the school have joint sessions in the observation room.
It allows normal lessons to be watched and discussed without disturbing the natural flow of things, said Mr Chedburn.
"I don't want teachers to worry about lessons going perfectly," he said. "An average lesson has highs and lows. I want teachers to see how their colleagues respond and get pupils back on task."
The school is now introducing cameras into the gallery to film lessons so that teachers can watch and analyse their own performance.
Next week, all 32 students completing their maths PGCEs at Newcastle University will use the room.
It has also hosted middle leaders from schools across Northumberland for interview training to help them prepare when applying for promotion.
Mr Chedburn believes other schools could set up a similar facility for less than pound;20,000.
As well as on-going training of staff, the gallery is also sometimes used for observations of teachers as part of their regular performance management.
"When teachers are being observed it's very difficult not to be distracted," said Mr Chedburn.
"Teachers constantly second guess what is going on in the mind of the observer, what different facial expressions mean, what they are writing down. With the gallery, teachers don't have that problem. It's easy to forget it's even there."
But Christine Blower, general secretary of the NUT, said: "It would be worrying to see it proposed as a `bright idea' for all schools. Systems of observation must be used with caution and with the agreement of staff. The potential for covert and inappropriate surveillance of pupils and staff is obvious."