Schools are struggling to get enough money to run camps because parents have realised they do not have to pay, principals say.
They say the government's school donation scheme has made parents more aware of the rules for payments.
The rules, which have been in force for several years, say schools cannot require payment for camps and overnight trips such as outdoor education or geography field trips that are part of the curriculum.
That applies to all schools, regardless of whether they are in the donation scheme which pays decile 1-7 schools $150 per child if they agree not to ask for donations for anything other than camps and overnight trips.
The president of the Principals Federation, Perry Rush, said principals were telling him that collecting contributions for camps and field trips had become more difficult.
"I've heard of a growing number of schools that have had challenges winning the funding that they have in the past to support camps," he said.
Rush said his own school, Hastings Intermediate, is in the donation scheme and was recently forced to postpone its annual camping trip.
"We simply didn't get the uptake in terms of payment that we've had in previous years so we've had to say to the community it's not happening at the end of this first term," he said.
"We've postponed it to the end of the year and we're very much hoping that our community will come on board as the year progresses and we'll see more donations."
Rush said the camp cost about $150-180 per child and the school had never had to cancel a camp before due to lack of payment.
He said fewer than half of families had paid and though they did not give a reason, he believed it was because parents now knew the payment was optional.
"Schools are starting to become quite concerned that with the new school donation scheme, and for those schools that have opted into that scheme, that parents have become much more aware that a school camp is a matter of donation. That a school, if it's part of the curriculum, can't enforce that payment."
Rush said principals were worried about the future of camps.
"This is a really important experience for young people. It's outside of the class, it's about taking risks, being supported in the environment, this is a really key part of being a Kiwi kid."
The principal of Waimea College, Scott Haines, said the number of parents willing to make donations for camps and field trips appeared to be dwindling and principals were worried.
Haines said he was not aware of any cancellations, but principals feared they would have to cut back on what they offered next year.
"There is considerable concern about what 2021 may look like if the number of donations are not what folk need to run programmes."