Wellington High School Old Girls' Association applied to Historic England to have their old school, which in modern times was New College, listed.
It has now heard that Oliver Dowden, Secretary of State for Digital, Culture, Media and Sport, has decided to add it to the list of buildings of special architectural or historic interest, and it has been Grade Two listed.
"It's wonderful news and we are absolutely delighted. It is such a beautiful building," said Pat Hayward, the association secretary.
"It's a great thing for Wellington and a great thing for the borough."
The effect is to give legal protection from demolition, extension, or significant alteration, without special permission from the local planning authority.
This means the ruling could have implications for the regeneration plans that Telford & Wrekin Council – which recently bought the site – has been working on.
Although a masterplan for the overall site has not reached planning application stage, ideas floated in the past have included a mix of housing, a special school, supported accommodation, offices, and small businesses.
Councillor Lee Carter, Cabinet member for finance, commercial services and the borough economy, said: "We welcome the listing of the Edwardian part of the former New College site. It means that part of the site is now protected.
"In the meantime we continue to work with partners and stakeholders to develop a plan for the wider site."
A merger of New College – a sixth form college – and Telford College of Arts and Technology, saw New College students transferred to the TCAT site from September 2018.
Pat said as far as she knew the Wellington building was now empty.
"It looks very sad," she said.
In giving reasons for granting listed status to the building, which dates from 1912, heritage body Historic England cites architectural interest, describing it as "an accomplished neoclassical design with a striking principal façade which survives unaltered, and incorporates sculptural embellishment," and also historic interest, as it is "by a regionally notable architect, F H Shayler, who made a significant contribution to the building stock of the area."
Pat had prepared a "just in case" application for listed status in 2017, but it was not submitted, and then as the association heard of various redevelopment plans being canvassed, she did submit an application on behalf of the association last year.
"We just took it upon ourselves to do it. We decided we should do something to save this treasure."
Pat said although nobody had suggested knocking down the building, "you never know."
In making their case for official protection of the building, the association drew in part on a booklet by Marjorie McCrea and Beryl Brown titled "Memories of Wellington Girls' High School."
When opened in 1912, the school educated both boys and girls, in separate parts of the building. The boys moved out in 1940 and it became an all-girls school.
Mrs Hayward attended as the young Miss Pat Lewis.
"I was at Wellington High School for Girls from 1968 to 1973, and my year was the last year to go all the way through the school. In 1975, wherever you were in the school, you were transferred to Ercall Wood.
"Then it became New College."