More than 320,000 primary children and Leaving Certificate students returned to their classrooms this morning for the first time since they left them last December
They included more than 260,000 of the country's junior primary school children, as well as 60,000 Leaving Certificate students.
Special schools also moved to full capacity today, having for the past two weeks welcomed just half of pupils on alternate days.
Students have lost a total of 33 days of in-school teaching during the closure.
The Teachers' Union of Ireland (TUI) urged vigilance to ensure that the return is safe and sustainable.
The union has said that it is "absolutely imperative that all safeguards and measures to protect health and safety in schools are adhered to".
TUI General Secretary Michael Gillespie has warned that while the preference of teachers is for a return to face-to-face teaching and learning "there is understandable anxiety among all in school communities, particularly given concerns around new strains of the virus".
The TUI has said it will not tolerate breaches of key safety measures in workplaces.
As the schools reopen, trade unions representing school workers, as well as public health authorities will be closely monitoring data on Covid-19 cases in schools and transmission rates.
Speaking yesterday on RTÉ’s This Week, Health Service Executive Chief Clinical Officer Dr Colm Henry reiterated that while there was no such thing as zero risk, the evidence to date showed that child-to-child transmission was not a major driver of the virus.
However, Dr Henry said he understood the concerns around the new B117 variant, first detected in the UK.
The variant, which is now the dominant one here, has been shown to be considerably more transmissible.
This has contributed to heightened concern around safety among teachers and other school workers, and some parents.
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The HSE has resumed the publication of weekly data related to schools. The latest data shows that five cases of the virus were detected in special schools during their first week and a half open. Three of the cases were in adults and the remaining two in children.
Teachers and other school staff who are pregnant or in a high-risk health category or over 60 years of age will remain working from home for the time being. This is likely to cause staffing difficulties in some schools.
The pregnancy clause will affect the number of teachers available to return to the classroom in particular, because Ireland's teacher workforce is predominantly female, and very young.
A significant proportion of Special Needs Assistants are over 60.
A target date of 15 March has been set for the return of all remaining primary school children, as well as fifth year students at second level, with the return of remaining second level students proposed for after the Easter break.
But the Government has indicated that these targets could be postponed if rising Covid-19 transmission rates make it advisable to slow down the phased reopening.
There are mixed views on the wearing of face masks among children at primary level.
Primary teachers union, the Irish National Teachers' Organisation (INTO), has said that while it favours the mandatory wearing of masks for all in a school setting, including pupils, it is not calling for this because of the absence currently of supporting public health advice.
The Department of Education has said that mask wearing is not recommended for children aged 12 and under, at this time.
However, it also pointed out that the Health Protection Surveillance Centre (HPSC) keeps infection prevention and control measures under review in all sectors.
While some primary schools have written to parents seeking to encourage the wearing of face masks by pupils, school principals in other schools have said that children, especially small children, would have difficulty wearing masks correctly.
Deputy Chief Medical Officer said there will be ups and downs over the coming weeks as schools reopen and public health teams have a difficult few weeks ahead.
However, speaking at tonight's NPHET briefing, Dr Ronan Glynn said that we know that the school communities "all got behind this" in September and made it a success between then and December.
He said he was confident it could be a success again, but his biggest concern is not necessarily what happens in the schools, but that everyone "buys into the measures" beyond the school community.
Dr Glynn said parents should wear masks while dropping their children off at school.
It is not formally recommended that children under 13 should wear a mask, he said, but any child who wants to wear a mask should not be discouraged.
Dr Glynn said the topic of children wearing masks will be discussed by NPHET this week, although he is not necessarily anticipating a change.