Coronavirus: Almost 40% did not attend reopened schools
Almost 40% of pupils invited to attend reopened schools in Wales did not go, Welsh Government figures show.
From 29 June, pupils returned in phases for "check in, catch up and prepare" sessions ahead of a full reopening in September.
Numbers attending at any given time have been limited because of social distancing measures and parents have not been fined for non-attendance.
The Welsh Government said it understood families' concerns.
Most schools in Wales closed ahead of lockdown on 20 March but some remained open as "hubs" for vulnerable children and the children of key workers.
Out of the estimated 24,112 vulnerable children in Wales, weekly attendance at school hubs between 24 March and 26 June ranged from between 0.9% and 6.3%, official figures show.
Vulnerable children are classed as children with a social worker or special educational needs.
When it was announced that schools would reopen from 29 June, Education Minister Kirsty Williams said it was important to ensure "equity" by giving all children the chance to "catch up" and prepare with their teachers ahead of the summer holidays.
At least three-quarters of Wales' schools only returned for three weeks from 29 June but the summer term was extended by a week for pupils in Conwy, Pembrokeshire and Powys.
In the three weeks between 29 June and 17 July, Welsh Government figures show that 61% of pupils that were invited to attend at least one weekly session did go to school.
But, in the first week, 37.1% of pupils invited to attend did not go, rising slightly to 38.7% in the second week and again to 41.5% in the third.
Flintshire and Swansea councils have not provided any data, and schools on Anglesey did not reopen until the last week of the summer term because of a number of positive coronavirus tests linked to the 2 Sisters poultry factory in Llangefni.
All state schools in Wales will reopen to all pupils in September with limited social distancing within "contact groups" of about 30 pupils.
But parents will not be fined if they do not send their children back.
Plaid Cymru said the figures were "not surprising", suggesting it was because parents and pupils "questioned the value" of returning for just a few days and were "fearful" of the virus.
Meanwhile the Welsh Conservatives suggested low attendance could be down to confusion over which councils would keep schools open for a fourth week before the end of term.
An education expert said the figures meant a "very large number of pupils will be starting school in September having had no face-to-face teaching for six months".
"Making up for lost time won't be straightforward, and our teachers will need time to rebuild relationships, establish new routines and identify bespoke learning support for pupils," said Gareth Evans, director of education policy at University of Wales Trinity Saint David.
A Welsh Government spokesman said: "Wales was the only UK nation where all pupils were given the opportunity to return during the summer term, to catch up with their teachers and classmates and prepare for September.
"However, we understand families' concerns and were clear attendance was not compulsory.
"We have provided guidance for schools so all pupils can return safely in September."