Parents are protesting at the Urdd Eisteddfod in Ceredigion over a decision to reject school reorganisation in Cardiff.
First Minister Carwyn Jones blocked plans to shut an English-medium primary and replace it with a Welsh-medium one.
Supporters of Ysgol Gymraeg Treganna in Canton say a "clear and definitive" answer is needed.
The assembly government said the reorganisation would not have improved education provision.
The protest is taking place during the youth festival at the National Trust estate at Llanerchaeron.
Cardiff council had proposed closing Lansdowne Primary and transferring pupils there to nearby schools, and moving Ysgol Treganna to the vacated building as part of a plan to cut surplus English-language places.
Treganna currently shares a site with Radnor Primary, but parents say Treganna is heavily oversubscribed and has more than 200 pupils in a space designed for 167.
A statement released on behalf of the protesters, which includes members of the Welsh Langauge Society, said Treganna's sister school Tan-yr-Eos was opened in 2007 on a site shared with Ninian Park School with the promise it would only operate for two years while a permanent solution was found.
It said Mr Jones's decision "gives no hope and no solutions to parents and pupils at Treganna and Tan-yr-Eos who have simply no idea what will happen to their children and their younger siblings in the coming years.
"As the first minister cites the detrimental effect of split-site education on pupils at English medium schools as grounds for refusal, this is already happening at Ysgol Treganna and Tan yr Eos, with brothers and sisters being split up and bused around for classes and sports lessons."
The protesters will wear the "Welsh Not" symbol at the protest in reference to the practice of making pupils who spoke Welsh in some schools during the 19th century having to wear or carry a piece of wood with WN ("Welsh Not") inscribed on it.
On Saturday, former Plaid Cymru leader Dafydd Wigley said the assembly government's decision contradicted its own Welsh-medium education policy.
In a statement at the weekend, the assembly government said: "Whilst it's the local authority's responsibility to identify alternative proposals that would provide a solution which will meet the needs of all pupils in Canton, ministers have asked officials to provide whatever support that can lawfully be given to the local authority to assist them to meet that responsibility.
"We recognise that any solution must ensure decent conditions and provision for children educated through both the medium of Welsh and English in the area.
"It is important that overcrowding in any schools in the locale should not happen as a consequence of any proposed solution."