Teachers say unhealthy food parents put into childrens' lunch boxes was one of the challenges faced and many were reluctant to be the "food police".
An Education Review Office report on good nutrition and physical fitness at schools found the challenges ranged from competitive parents on the sidelines to the food parents gave children to bring to school.
Early childhood centre teachers told the ERO one of the biggest challenges was unhealthy food in children's lunchboxes, and a reluctance to be seen as the "food police".
That was also a concern echoed at the primary and secondary school level, where leaders said the proximity of local dairies and takeaways were often visited by children "or parents provided unhealthy food for their children to eat at school".
Schools also reported parents' own sporting ambitions for their children could have the opposite effect - and put a child off.
"Parents could be competitive and have inappropriate expectations, and some behaved poorly on the sidelines of their childrens' sports games. This behaviour could be discouraging to some children and meant that coaches and managers were not able to give the children their full attention."
The ERO report was commissioned by the Government in 2015 as part of its anti-obesity strategy. It surveyed how well 202 early childhood centres, 46 primary schools and 29 secondary schools were dealing with good policies around food, nutrition and physical activity.
Food and nutrition is part of the school curriculum. Last week, Health Minister Jonathan Coleman and Education Minister Hekia Parata announced the appointment of Professor Grant Schofield as the Ministry of Education's first Chief Education Health and Nutrition Adviser.
Coleman said there were many initiatives under the childhood obesity strategy to help schools, such as Health Promoting Schools which involved the community. Many also had policies such as "water only" schools. However, regulating the contents of children's lunchboxes would be a step too far.
He said it was up to childcare centres to decide how to approach the issue.
"I think once you're telling people what to put in their kids' lunch box, that's quite a step and some parents would find that offensive. I think there's ways of getting messages across rather than sending notes home saying 'don't send them in with a chocolate slice again'. It's actually a parents' prerogative to assemble a balanced diet for children."
Peter Reynolds, the chief executive of the Early Childcare Council, said it was a difficult position.
"Nobody wants to be seen as the food police. Nobody wants to send a note home in red ink telling parents they don't know how to feed their kids.
"But at the same time, most childcare centres will go out of their way to provide nutrition information to families so families have an understanding of what young pre-school age kiddies need to grow and develop."
He said many centres held seminars on affordable healthy food for parents. Some centres cooked for the children themselves, but changes to the Food Act meant more kitchens were closing down because of the extra administrative cost.
The report found secondary schools which put a high emphasis on competition in sports reduced the chances for other students to take part.
That was rejected by Secondary Principals' Association president Mike Williams, the principal of Pakuranga College, who said most schools tried to do both.
"You can do both. Yes, at the top end it is very competitive in secondary schools and some are very, very competitive.
But the majority of schools would be like our school - you want kids to be involved and you want students to try new sports."
The ERO report also found:
• While 87 per cent of early childhood centres were doing well on promoting good health, that dropped to 74 per cent of primary schools and 62 per cent of secondary schools.
• Work demands meant parents were not as involved in the school as in the past, leaving teachers under pressure to coach and manage sports teams.
• Senior secondary students said a lack of time due to study and social demands meant they did not play sports.
• Many schools contracted the tuck shop out to private providers which were "profit-driven" and "tended to be most interested in what would sell" rather than healthy choices.
Williams said most tuck shops offered healthy food and many schools had adopted policies such as banning soft drinks. However, they could not stop students buying from local dairies or takeaways.
Labour's education spokesman Chris Hipkins said it was understandable schools would contract out the tuck shop, but it would mean a decrease in quality.
He was also concerned at the finding students didn't have the time to participate in physical activity.
"It shows how 'high stakes' the pressure around NCEA has become."
One of the schools the ERO looked at was Kaiti School in Gisborne.
Nearly all students were Maori or Pacifica and it offered bilingual education in Maori and Tongan.
It got the tick from the ERO for its clear plan on health - and the board of trustees' decision to employ one fulltime sports leader and two part-time sports staff. It also subsidised fees for Saturday sports and specialist sports.
The sports leader mentored other teachers and all teachers took part in sports and physical education - including sessions on keeping safe in the surf, dancing and cycling.