The New Zealand School Trustee Association is concerned about the situation because in accordance with legislation, a voting paper received after voting closes at noon this Friday would be invalid, unless the voting paper was posted and the date stamp on the envelope showed it was posted before noon on Friday.
Association president Lorraine Kerr said any voting papers that were postmarked by noon on election day and arrived within five days could be counted.
However, New Zealand Post has confirmed it could not guarantee all envelopes would be postmarked.
The erratic nature of postal deliveries meant voting papers could take up to a week to arrive.
For those envelopes that were not postmarked, returning officers would not have any evidence the envelope was posted before noon on election day, she said.
Most schools had the minimum number of candidates to fill the available positions on their board, and did not need to have an election.
But 1151 schools across the country, including some in Otago and Southland, had more candidates than spaces available, and would need to have an election.
In light of the situation, she encouraged voters to hand-deliver their vote before noon on Friday, to guarantee it would be counted.
She believed the situation would mainly affect rural schools, where it was not always convenient for parents to go to their school and drop off voting papers.
''The worst case scenario is some valid votes may be left out of the count because they don't have a valid postmark.
''Parents have a right to vote and what we don't want to do is shut the door on that.
''We're working tirelessly on trying to predict what the impact will be in terms of how many schools this will affect.
''And then we will work out what we will do with the late votes - or votes that aren't postmarked - within the confines of the legislation.''
Mail general manager Matt Geor made assurances New Zealand Post was making every attempt to ensure voting papers returned by mail would receive a date stamp to ensure they were counted.
He said there was no legislative requirement for New Zealand Post to date stamp domestic mail, and its priority was to deliver letters within three working days.
''If we encounter difficulties with a date stamping machine - which can happen on occasion due to the nature of technology - letters are processed without a date stamp to ensure they are delivered within target, rather than delayed.
''However, we can say that the vast majority of mail is date stamped.''
