Second-level schools in Dublin are being particularly badly hit by teacher shortages because of the housing crisis across the city.
There is a general lack of qualified teachers in a wide range of subjects, including Irish, home economics, and maths.
Science - particularly physics and chemistry - and modern languages are also being badly hit by the shortage.
Rising enrolment in second-level schools is creating the demand for more teachers.
However, many qualified teachers have taken career breaks to work in the Middle East where they can build up deposits for homes on the back of tax-free salaries.
The problem of filling vacancies is particularly acute in the capital, because of the difficulties staff face in trying to find affordable accommodation on a teacher's salary.
Delegates at the annual conference of the National Association of Principals and Deputy Principal (NAPD) offered anecdotal evidence of teachers turning their back on opportunities in Dublin to take up positions elsewhere.
A large, fee-paying school in the capital advertised in recent weeks for a maths teacher and a science teacher.
However, it received no applications for the science post, and only one for the maths jobs.
In another typical example, a south-Dublin school recently wrote to parents to advise that, despite several attempts, it couldn't find a home economics teacher to cover for a maternity leave.
NAPD president Cathnia Ó Muircheartaigh raised the issue directly with Education Minister Richard Bruton at the conference.
Mr Ó Muircheartaigh warned the minister the situation was so serious that "we could be looking at cancelling student activities or being unable to release teachers for professional development".
NAPD director Clive Byrne told the Irish Independent that the high cost of housing was affecting people's willingness to live in the capital.
He said teachers were being "attracted to positions around the country other than Dublin, where accommodation was more available and affordable".
Mr Byrne referred to the UK and the "London allowance" which is paid to public servants such as teachers to compensate for higher living costs in the city, but there was no such system in Ireland.
A number of initiatives are already in place, or under consideration, to address teacher shortages, such as encouraging retired teachers back into the classroom, but they are playing catch-up with the problem.
Mr Bruton said yesterday that among the new measures being considered was the use of the Springboard upskilling programme to encourage homemakers, with third-level qualifications, to consider returning to the labour market as a teacher.