The Secondary Principal's Association says efforts are being made to stop incompetent teachers getting through the system, but it may take time.
The association is concerned about the poor literacy and numeracy skills of some teachers and that some can't even do primary school maths.
It's believed 174 cases have been referred to the Teachers Council over the past five years, but only one teacher has been deregistered.
The Dominion Post on Tuesday reported that secondary principals have found some teachers so lacking in literacy and numeracy they can't write adequate reports or do primary-school maths.
Association president Pat Walsh told Nine to Noon that incompetent teachers ended up in the classroom because they slipped through the system.
The association says it's been told that some teachers needed to undergo remedial work in order to write basic reports.
Some teachers are not able to cope with mathematics text books for the year nine and 10 level.
The Teachers Council says most of the 174 referred teachers are unlikely to work in the profession again.
Education Minister Anne Tolley says law changes brought in last year should stop incompetent teachers from being re-employed.
She says this is because conditions are imposed on their practicing certificates.
However, the Secondary Principals Association fears that these teachers could be hired when schools are desperate for staff.
