The jury at Swansea Crown Court heard the girl, who cannot be named due to legal reasons, carried a knife to school every day and had moved from another secondary school to Ysgol Dyffryn Aman.
The court also heard a knife had been found in her bag earlier in the school year.
"There must have been flags there that were missed," Mr Hopkin said.
While the school has been supportive, he added, he criticised the response from the Welsh government and local education authority (LEA).
"We just think it's ridiculous this inaction by the Welsh government, and by the LEA, but I think it's got to be led by the Welsh government first," he said.
"They just seem to be kicking the can down the road all the time."
'We need action'
The girl's first trial at Swansea Crown Court started in September 2024 but collapsed due to "an irregularity within the jury" which meant the verdict was not reached until February this year after a second trial.
But Mr Hopkin added: "They didn't need to have the verdict for this to tackle the rise in poor behaviour.
"In the county council, nobody has offered any support other than the counselling that came through the school at the beginning, that's carried on.
"Schools don't feel that empowered. We need the action, the discussions are done."
Both Ms Hopkin and Ms Elias have called for changes since the court case ended in February.
There have also been calls for a review of safety in schools from politicians, including from Plaid Cymru Senedd Member Cefin Campbell, whose brother eventually restrained the attacker on the day.
UCAC teachers' union said it had been a "year-long battle" to get around the table to discuss the issue, with its members raising concerns about "danger to their lives".
Another union, NEU Cymru, welcomed discussions but said "significant investment" was needed "quickly".
"This extreme incident is the tip of an iceberg. There is a myriad of issues around it," said the union's interim secretary Nicola Fitzpatrick.
She added there was ongoing discussion over a need for a shared protocol to avoid inconsistent guidance and support across Wales.
BBC Wales offered Welsh government's cabinet secretary for education Lynne Neagle an interview, but a statement was sent which said her thoughts were with the victims of this "terrible attack" and their families.
"Making sure learners and staff feel safe and supported in our educational settings is essential and we are taking action to address declining behaviour in schools and colleges," she added.
"The recent Estyn report highlights the issues in our secondary schools and we will be taking forward the recommendations in the report and discussing these, along with the findings of the violence and safety in schools roundtable at our behaviour summit this month."
Glynog Davies, Carmarthenshire County Council's cabinet member for education and Welsh language, said the regional safeguarding board had commissioned a multi-agency, independently-led review.
He said the review was now underway to ensure good practice was highlighted and shared and that any lessons learnt were identified to improve practice and systems in the future.
Davies added the local authority had supported the victims of the incident as well as the wider school community, which had been been "well received" by many.
