Llanfihangel Rhydithon primary school in the Dolau area of Powys, which has 36 pupils, will close at the end of August in a bid to save Powys County Council £76,000.
The fate of the primary school, which is between Llandrindod Wells and Knighton, was sealed by councillors at a cabinet meeting on Tuesday.
The cabinet were looking at the 200-page official objections report after making an initial decision in September after a consultation last year.
They listened to arguments by local county councillor Hywel Lewis, as well as Liberal Democrat group leader Councillor James Gibson-Watt, against the closure.
Howeverthe cabinet voted unanimously in favour of closure, meaning pupils will have to attend an alternative school from September.
Councillor Lewis pointed out that in collaboration with Llanelwedd primary school, Llanfihangel Rhydithon had been going from “strength to strength.”
Councillor Gibson-Watt wanted to see the cabinet “pause and reflect” on the current school reorganisation programme.
He said: “The school is more or less full.
“We’re not talking about educational standards, those are very good, it’s working very successful with another school.
“So, what’s driving the idea of closing this school?
“The truth is because it’s a small school.
“That seems to me to be the overwhelming driver, all small schools would be in line for closure in due course.”
“This will have massive implications and long-term consequence for the county as a whole.”
Councillor Gibson-Watt added that the cabinet had not decided what they wanted to do to the Ysgol Calon Cymru catchment area of Llandrindod Wells and Builth Wells.
“Yet today you’re deciding to close one of its feeder schools, strategically I don’t understand how on earth it fits in,” said Councillor Gibson-Watt.
Education portfolio holder, Councillor Phyl Davies said: “On surplus capacity and there not being any in Dolau, that is correct.
“But not of the catchment.”
He believed that rectifying surplus school spaces across catchment areas was something Estyn, the Welsh school inspectorate, had wanted.
School transformation manager, Marianne Evans said: “The option to federate with Llanelwedd came up in the consultation, it is not legally possible to federate a Church in Wales school and a community primary school.”
“We are looking at potential options for Ysgol Calon Cymru and have recently finished an informal engagement about the provision.
“Comments have been made why we haven’t included all the primary schools in a formal area review
“We have to look at where the problems are and the highest priority to address.
“That is in fact Ysgol Calon Cymru itself.”
Councillor Hywel Lewis said that Llanfihangel Rhydithon had made it “quite clear” that they were prepared to become a church school to accommodate a federation and had been in touch with the diocese of Swansea and Brecon on that point.
“The process was started but the education department stopped it,” said Councillor Lewis.