The NASUWT teaching union has called for the implementation of a new school curriculum to be delayed to help teachers and students recover from the pandemic.
A new Curriculum for Wales is set to be introduced next year after it was recently approved during a Parliamentary vote.
However, the NASUWT union has said schools are being asked to deliver the curriculum without enough time to prepare.
They added that the timescale was putting the very success of the new system at risk.
Dr Patrick Roach, General Secretary of the NASUWT, said: "These are landmark reforms which will have a profound and lasting impact on the futures of a generation of children and young people.
"It is therefore absolutely vital that these changes are introduced in a way which commands the confidence of teachers, pupils and parents.
"Teachers have been on the frontline during the Covid-19 pandemic, coping with unprecedented pressure to support pupils both in school and through remote learning.
"The Welsh Government must give teachers sufficient time, training and resources to ensure the introduction of the new curriculum runs smoothly.
"We will continue to take all steps necessary to ensure the strength of feeling of our members on this is heard loud and clear by ministers."
The new curriculum will replace the national curriculum, which had been a core part of Welsh education since 1988.
Six new Areas of Learning and Experience (AoLEs) will be introduced as boundaries between traditional subjects are abandoned.
Although it will focus on three core areas: literacy, numeracy and digital competence.
In response to the NASUWT's concerns, the Welsh Government said reform of the curriculum remained a "key priority".
A Welsh Government spokesperson said: "We have been clear that curriculum reform remains a key priority and must continue.
"The national continuity plan we’ve published to support schools in response to Covid-19 shares the same principles and philosophy as the new Curriculum for Wales.
"Both are closely aligned so that schools can support pupils’ wellbeing, knowledge and skills.
"We are providing record levels of investment in staff professional learning to further support schools as they prepare for the introduction of the new curriculum in 2022."