The UCAC teaching union said coronavirus cases in schools could force whole class “bubbles” into self-isolation over the holidays.
The union said schools should close on 11 December and move lessons online.
The Welsh Government said schools would only close if there were clear public health reasons.
In a letter to Education Minister Kirsty Williams, the union said there were “serious concerns” among UCAC members “that pupils and students will continue to attend an educational setting a week before Christmas Day”.
Most schools are due to break up in the week starting 14 December, but if positive cases are found in pupils’ class bubbles, children would have to self-isolate for two weeks, meaning some may have to isolate on Christmas Day.
“If a pupil or student tested positive with Covid-19 during the last week of term, it would mean that the whole bubble would have to self-isolate, preventing them from joining their extended family for Christmas,” General Secretary Dilwyn Roberts-Young said in the letter.
“The same could be true for the education workforce.
“In addition, Test, Trace and Protect could continue to contact school and colleges leaders on Christmas Day, which is totally unacceptable.”
The union said that closing schools early would give families “greater reassurance” that they could get together to celebrate during the five day Christmas travel rules.
“We are acutely aware of the need to ensure continuity of learning for all pupils and students in Wales, particularly in the wake of the disruption earlier in the year, which has continued to some extent since September,” the letter said.
“We believe that making an early announcement on this would not only allow the workforce to prepare thoroughly for that week, but also enable families to make alternative childcare arrangements as needed.”
It said a survey of members prompted a thousand responses with 75% in favour of online learning in the final week of term.
The union has also asked the minister to consider an alternative or parallel move to offer Covid tests to staff members once contact with pupils and students has come to an end, from 18 December onwards.
Some local authorities are also believed to be in favour of closing school sites a week early, though provision for children of key workers and vulnerable children would be one potential concern.
The latest Public Health Wales figures show that 50.3% of primary schools and 87.9% of secondary schools have had at least one Covid-19 case since the start of term.
There were 414 positive tests amongst school pupils or staff in the week up to 25 November.
A Welsh Government spokesman said they were in regular talks with councils and unions about schools.
“Our priority remains ensuring that all children and young people’s education continues with as little disruption as possible,” he said.
“We would expect face-to-face learning to continue as planned until the end of term unless there was a clear public health reason for not doing so.”