The union for primary school teachers says some schools do not realise they are breaching the Employment Relations Act by trialling beginning teachers with fixed-term contracts.
The Educational Institute (NZEI) wants schools to sign up to a charter that commits them to giving new teachers the support they need to get full registration.
It said only 15 percent of newly graduated teachers get full-time, permanent jobs. Others get genuine fixed-term positions such as those created by teachers taking parental leave.
However, the institute said some schools hire beginning teachers on fixed-term contracts simply so they can try them out, and that is illegal.
"The charter guarantees that any beginning teachers the school employs will receive the induction and mentoring support they are entitled to.
"The school also commits to never employing new or beginning teachers on a trial or otherwise illegal fixed-term basis," the NZEI said.