Schools in Wrexham will delay opening classes to children aged three to seven due to concerns over coronavirus rates.
Foundation phase pupils are due to start a phased return to school from the Monday after half term.
Wrexham still has the highest rate of Covid infections in Wales, and will not be sending children back until Friday, 26 February.
Anglesey, Denbighshire and Gwynedd are also delaying the immediate return of younger pupils.
The Welsh Government announced last week that primary schools would begin the process of reopening for the first time since Christmas on 22 February.
Wrexham council said: "Although coronavirus levels are falling in Wrexham, the figures are still very high, so the council and headteachers are carefully considering plans, while continually monitoring the local situation.
"In light of this cautious approach, children will not be returning to schools in Wrexham until Friday, 26 February at the very earliest - depending on local coronavirus levels after half-term."
Wrexham and Flintshire still have six of the 10 highest localised rates in Wales, when they are broken down by community - or what are known as middle layer super output areas (MSOAs).
Although these case rates too have dropped back in recent weeks.
The highest in Wrexham - Wrexham West - has seen 35 actual positive tests over the last week.
There are local hotspots too in Holywell and Bagillt and part of Connah's Quay in Flintshire
Flintshire council, which currently has the second highest case rate in Wales, said it would review a decision to reopen schools on 22 February "should the public health situation become a cause for concern".
Schools on Anglesey, which has the third highest rate of coronavirus infections in Wales, said it would also be delaying pupils returning, phased from Thursday, 25 February.
"Our aim is that all foundation phase pupils are back in their classrooms by Wednesday, 3 March at the latest," Anglesey officials said.
Meanwhile Denbighshire said its primary schools would reopen to foundation phase pupils from Wednesday, 24 February, as long as "all necessary safety measures" are in place.
In Gwynedd, the council has confirmed the Monday and Tuesday of that week will see schools remain closed for staff to prepare for pupils' return.
"Gwynedd's Education Authority has met with head teachers this week to ensure they are fully aware of the latest development and our officers continue to offer help and advice regarding the planned phased opening for foundation phase pupils during the week beginning 22 February," a spokesman said.
Free school meals and school transport will be available for all eligible pupils when those schools reopen, according to the Local Democracy Reporting Service.
Children classed as vulnerable and those of critical workers have been able to remain in school during lockdown. No timetable has yet been given for the return of other children.
In an open letter to headteachers, Education Minister Kirsty Williams said the Welsh Government had "always said" schools should be "last to close and first to open" when safe to do so.
She said "much lower" rates of transmission of Covid-19 and advice from its Technical Advisory Group had allowed the "partial and phased return to face-to-face learning in schools".
Ms Williams explained there would be some flexibility to the arrangement over the first week, but with all foundation pupils in school by the end of the week.
Any further phased return for other year groups will be considered as part of the next 21 day review on 19 February.
The Welsh Government has also issued detailed guidance on the return to classes, with teachers told they will be expected to face masks in some circumstances.