A shortage of qualified teachers is forcing students to turn to grind schools, increasing divide between the "haves and have-nots", according to the National Association of Principals and Deputy Principals.
The NAPD says many second level schools opened this year without a full complement of staff.
It says there were severe issues in Galescoileanna, due to the lack of suitably qualified teachers, particularly in the areas of languages, STEM subjects and Home Economics.
The association is calling on the the newly appointed Minister for Education and Skills, Joe McHugh, to take urgent action to address the problem.
It is also urging the minister to tackle what it calls "initiative overload" which, it says, is crippling school leaders.
Over 500 second level school leaders are meeting at the NAPD annual conference in Galway today.
NAPD President Mary Keane, has said that: "Teaching supply is the biggest crisis our sector faces. It is hugely important that we have fully trained high-calibre teachers in our classrooms.
"We heard last week that there will be an increase in teachers posts and SNAs, however, as we embark on yet another school year, the lack of suitably qualified teachers is still a troubling concern with many schools opening without a full complement of staff this year.
"This crisis also creates a socio-economic divide in many schools. With substitute teachers or fully qualified teachers in a number of key subjects impossible to find, parents, who are in the fortunate position to be able to do so, are supplementing their children's education with grind schools.
"This is creating an unfair divide between "haves and have-nots" which undermines the value of our public education system".