Shropshire Council has recommended the closure of Wakeman School in 2013, due to falling pupil numbers making it "financially unsustainable".
Lib Dem councillor Roger Evans said the plans would save only £300,000-£400,000 each year.
"There's 30,000 pupils in Shropshire. That's £10 per child going extra to every school in Shropshire," he said.
The proposals, announced by the Conservative-led authority on Wednesday, require approval by the cabinet group on 7 September.
From September some 240 pupils will be on roll at the Wakeman.
Mr Evans said falling numbers over recent years were due to rumours regarding the school's future.
Plans to close the Wakeman have centred on finances, rather than improving educational standards.
In March Ofsted reported that the Wakeman was a good school, with a good capacity for sustained improvement.
Last week the school also achieved its best ever GCSE results.
David Taylor, director of people services at Shropshire Council, said the school's closure would financially benefit the county overall, without damaging the pupils' quality of education.
"This is not an easy part of my job. But at the end of the day I've got to do what's right for all the children in Shropshire," he said.
Mr Taylor said the closure of the Wakeman would make about £1.75m "available for redistribution" to other schools per year.
Mr Taylor added that £1.3m of that sum would "follow the child".
That represents the total allocation for pupils at the Wakeman, a large portion of which would be spent on the cost of educating the pupils at another school.
The difference between the two figures (about £400,000) covers the annual saving made through closing the building, and the associated operational costs, such as administration, heating and electricity.
The council said the pupils could be accommodated at other schools and identified Meole Brace as the preferred option.
The number of secondary pupils in Shrewsbury is expected to rise from 2020.
The council said had not ruled out building another school on the edge of town if required, possibly through the resiting of an existing school.
A Shropshire councillor has criticised plans to close a Shrewsbury secondary school.