The Teachers' Union of Ireland has warned that at least 1,200 new second-level teachers will be needed within five years.
The claim is based on the latest projections which indicate an increase of up to 60,000 in second-level numbers.
The union, whose annual conference begins tomorrow, said now is the time for planning to avoid potential overcrowding in classes.
Jim Dorney general secretary of the TUI said: "While the second-level age population currently totals around 340,000, the figure for the year 2020/21 is likely to be over 400,000.
"This may seem like a long time away, but the reality is that provision has not been made for our changing demographics."
There has been an increase of some 10,000 in the number of pupils attending our primary schools since 2000 and these will have to be accommodated within the second-level system, he added.
The number of students attending second-level is likely to stay static for the next five years, followed by a swift and unprecedented rise in the subsequent years, according to the TUI.
The union said the Government should seize the opportunity to lower class sizes, especially in the poorer areas.
A level of 15 pupils for each teachers could be maintained by employing more permanent full-time teachers as student numbers rise.
Mr Dorney said: "Put simply, classes at primary and second-level are already overcrowded.
"If this is not remedied in the near future we will be ill-equipped to deal with increased numbers later on."