Wales' education minister is facing calls to drop her plans to reopen schools at the end of this month.
In a letter seen by BBC Wales, teaching union UCAC urged Kirsty Williams to keep schools closed until September.
As plans to reopen primary schools in England were changed, the union said schools faced "mind-boggling" challenges.
But the Welsh Government said the change of policy in England would not affect their plans.
A spokesman confirmed the guidance on how schools in Wales should reopen on 29 June will be published on Wednesday as planned.
Most unions criticised last week's announcement that schools in Wales would reopen for all age groups for limited periods during the week.
Welsh Government officials pointed to comments by England's children's commissioner, who has called on the UK government and unions to "stop squabbling and agree a plan" to reopen schools in England.
Teachers representatives met Welsh Government officials to discuss the plan earlier.
Before the meeting, UCAC wrote to Mrs Williams saying schools were being asked to prepare for children to return "without knowing what arrangements will be in place to ensure their safety".
Opening in September "would be the wisest option" UCAC said, but allowing Years 6, 10 and 12 to return before the summer "could be workable".
"Under the current circumstances, the practical, logistical considerations involved in seeking to give every pupil the opportunity to return to school before the summer are mind-boggling," it said.
"UCAC believes that the risks the government is asking the school workforce to take are unacceptably high."
The union called for reassurance on a range of points, including the supply of personal protective equipment and coronavirus testing for pupils and staff.
'Guidance is too late'
Meanwhile, teaching union NASUWT said the guidance being published on Wednesday would be too late and was "not firm enough" in directing schools and local authorities to adopt safe working practices.
"This guidance should have gone out when the minister made the statement regarding return to schools last Wednesday," said the union.
"In the last week some local authorities have been putting together plans that may now be in conflict with the Welsh Government guidance."