Dublin principals are reporting issues in filling vacant posts, as many teachers opt for a more affordable lifestyle elsewhere in the country.
One school in south Dublin has had six teachers hand in their notice to relocate elsewhere.
The ASTI says many of its Dublin members are struggling to fill vacant posts before September, particularly in Irish, Maths and Home Economics.
Deputy General Secretary Diarmuid de Paor says other members of staff are being asked to fill the gaps
"They are asking teachers, for example, who have Irish but are not qualified Irish teachers to take some classes.
"They are qualified teachers, it's important to stress that but they may not have a qualification in Irish.
"Obviously at the beginning, (principals) will try not to do that for the honours Irish or Maths classes for Leaving Cert but they may find themselves with teachers who have a degree in Physics taking on Maths classes."
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