There has been a sharp increase in the number of children expected to need school places in the coming years.
Revised projections from the Department of Education show student numbers at second level will grow by 26% over the next 14 years, to 413,118.
Previous estimates had put the projected number for 2026 at around 383,000.
The figure was revised upwards after census figures revealed continuing high birth rates.
Head of the Teachers' Union of Ireland John McGabhann said the figures had changed because fertility rates, migration and 2011 census information had now all been factored in.