Universities have rejected Education Review Office criticism of their teacher-training courses.
In a report published on Monday, the ERO said 60 percent of the principals it interviewed said their new teachers were not ready to be in the classroom.
The report also found graduates from some universities felt better prepared than others, as did those from courses that involved more time in the classroom.
It called for an exit exam for graduating teachers, higher entry standards, and a push to attract the most academically able students.
Alison Kearney, chairperson of Council of Deans of Education, which represents the country's teachers' colleges, said the ERO report was based on responses from a small number of new teachers who trained during the peak of the pandemic, when many were not able to spend the usual amount of time in classrooms before graduating.
"We are concerned about ERO's use of a small-scale survey of less than 10 percent of new teachers and 12 percent of principals to make sweeping, system-level claims and to compare different education providers."
It was normal for people to feel unconfident when they started a new job, Kearney said.
"This is in keeping with research in other professional areas as well. Professions such as lawyers and engineers and psychologists, they all report similar things."
The ERO report did not provide enough evidence to warrant increasing the amount of classroom "practicum" time, Kearney said.
The Teaching Council required student teachers in a one-year teacher education programme - the norm for secondary teaching - to spend at least 80 days or 16 weeks in classrooms, and for those in three-year programmes the minimum was 120 days or 24 weeks.
Kearney also rejected the report's call for an exit exam for graduating teachers.
"Exit exams are not in keeping with evidence-based practice," she said.
"Examinations are really good if you're trying to measure rote learning or, at their best, useful for surface-level recall rather than deep understanding. So for something as complex as teaching, I'm sure that people would agree that examinations would be inappropriate and not really of value."
Universities already conducted a final assessment, which was mostly oral and covered complex areas of decision-making, Kearney said.
New teachers' confidence in the job and their decision to stay or go were influenced by many factors, she added, noting that surveys showed a large number of teachers were unhappy with high workloads, lack of support and their pay rates - all factors that had nothing to do with their initial teacher education.