James Evans has said it is "bitterly disappointing" to see the closure of another rural school in the area.
The Brecon and Radnorshire MS said people would "rightly be questioning" the decision from Powys County Council's Cabinet.
It was announced this week that Llanbedr Church in Wales School will be shut later in August this year.
The Liberal Democrat-led Powys County Council finalised the decision at a Cabinet meeting yesterday (Tuesday).
Mr Evans said: “It’s bitterly disappointing to see the closure of another rural school by this Council.
“Llanbedr has played a crucial role in the community, and this simply isn’t fair on the children who attend the school.
“People will rightly be questioning why the Lib Dems previously committed to halting school closures but are now doing everything they can to close rural schools.”
Llanbedr chair of governors, Elaine Lusted said the community was devastated by the decision, and was equally critical of the Lib Dem administration.
Powys Liberal Democrats have come under heavy criticism for their decisions on rural schools in the county since gaining control of the council last year under a Lib Dem/Labour ruling coalition.
Conservative County Councillor Iain McIntosh last year called for resignations from Lib Dem County Councillors, accusing the party of breaking its pre-election promise on rural schools.
Prior to the local government elections last May, the now leader of Powys County Council Cllr James Gibson-Watt said that people should vote for the Welsh Lib Dems if they wanted to see village schools "retained in the future".
He added that his party were "committed to seeing our rural schools kept open within new clustering arrangements in each secondary school catchment area."
In a Facebook post in November 2021, Welsh Lib Dem leader Jane Dodds MS, tagged the Llanbedr School Parents Group and said: "If the Welsh Liberal Democrats were to take control of Powys Council next year, we would halt all rural school closures.”