Some 50,000 new pupils entered primary schools around the country and 50,000 started secondary school as the new academic year began this morning.
A number of new subjects have been introduced for primary school children and changes have also been made to course structures at secondary level.
For the first time, children at primary school will study science. They will also take classes in social, personal and health education and in civil, social and political studies.
At secondary school level, changes have been made to the Junior Certificate science syllabus. However, the Association of Secondary Teachers, Ireland (ASTI) has instructed its members not to teach the new syllabus on health and safety grounds until the recommendations of the Task Force on the Physical Sciences are implemented.
The union said it supports the new syllabus but that the conditions for its safe and successful introduction have not been met.
The ASTI said the Task Force on the Physical Sciences had recommended a national programme be put in place this year to provide all schools with first-class laboratories and equipment and that they should be provided with school laboratory technicians.
In addition, the body said the idea of allowing schools the option to take the old or new syllabus was "educationally divisive" and would increase educational inequality as it placed schools whose science facilities were inadequate at a serious disadvantage.
Meanwhile, the Minister for Social and Family Affairs, Ms Coughlan, has announced the expansion of the school meals scheme. Funding has been doubled to €4 million and almost one hundred schools across the country have expressed an interest in setting up a new school meals scheme, the minister said today.
She said the ban that restricted the scheme to urban primary school had been removed in a bid to target disadvantage. It is expected more than 4,000 children will eventually benefit from free school meals.
On the roads today, AA Roadwatch said the September 1st return to school had not resulted in the "chaos" traditionally expected.
A spokewoman said traffic had been noticeably heavier in Dublin, Cork, Galway, Limerick and Waterford. However, she said some schools were not reopening until later this week, which may have meant lighter traffic today.