Teacher union the New Zealand Educational Institute says schools are struggling to find relief teachers to cover for sick staff.
Ministry of Education figures indicate there are 3000 fewer relief teachers now than there were a decade ago.
The NZ Educational Institute Te Riu Roa which covers primary and early childhood teachers says there needs to be a complete review of the relief teaching workforce.
Its president Mark Potter said there was significant pressure on relief teachers.
"There's a lot of demand out there and there just aren't enough of them to go around, so they're a workforce stretched very very thin, a workforce of increasing value but they're not being valued by the way we currently fund and resource the system."
Relievers have become more of a specialist teacher now with far more teachers needed on a part-time casualised basis, he said.
But the system is not geared to help relievers to ensure they continue to develop as a professional teacher which results in some opting to leave the profession, Potter said.
The pay structure also had salary caps on how much particularly kindergarten and primary teachers could earn which was a barrier to viewing relieving as a long term career option, he said.
Some schools were securing relievers before they actually needed them, Potter said.
"There are some that are booking well in advance, up to at least a term or nearly two terms just to make sure they have got somebody there, they can always change what they've got them for but it means that they definitely have that person.
"It's a bit of a joke that principals watch each other sideways now to make sure they're not helping themselves to your relievers but that's showing you just how few relievers there are out there."
A lack of relievers leads some schools to having to double up classes, while in other cases there is no option but to send children home because they cannot find a relief teacher to take a class, he said.
It was necessary to have a better plan for the entire teaching workforce to address issues such as pay, recruitment, development and retention, he said.