The ASTI has accused the Government of having "toyed with" with the teacher recruitment crisis, and offering merely "cosmetic" solutions that "lack ambition".
The General Secretary of the union, Kieran Christie, has said 82% of schools had no applications for vacant posts in the past school year.
Speaking on RTÉ's News at One he that "this year is as bad as ever", as measures introduced by the Government "haven't made a dent in the problem".
The union has warned that schools are under enormous strain and many may be "barely able to cope when they reopen shortly".
It published a survey conducted with RED C and described its findings as "shocking".
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No substitute teachers were available to cover for absent teachers in 87% of schools, while 77% had to employ unqualified teachers to ensure that classes were taught.
Mr Christie claimed that Minister for Education Norma Foley "has been inexplicably reluctant to make the changes necessary to fundamentally address the problem", and called on her to "change course".
"The minister has skirted around the problem," he said, and urged that she "belatedly realise that it is impossible to entice teachers to return from other countries on the promise of part-time or fixed term positions only".
He said that the ASTI has "continually pressed" the department but that its appeals have "fallen on deaf ears".
"The United Nations High-Level Panel on the Teaching Profession recently reported on the on-going and worsening crisis in teaching globally, and recommended that governments ensure equitable funding for education and sustainable investment in the teaching profession.
"Our government is falling short on both fronts. It cannot continue.
"The children of our country are paying an enormous price in this crisis and it will leave a long and bitter legacy."
