None of the inner-city group of 15 schools from Point Chevalier to Freeman's Bay and including Waiheke Island, were told about the charge so they could challenge it before introduction.
"It's not about the $90 charge, it's that we feel our parents pay council rates and schools should be able to access public parks without any cost."
The school is on a 1.6ha site and because of its limited green space uses other local parks for sports such as its cross-country race.
The school wrote to the mayor and Councillor Christine Fletcher, who is chair of the parks, recreation and sport committee.
Cr Fletcher said she was shocked to learn that schools were being charged because that was not the intention of the councillors who approved the new bylaw.
She said she would ask council managers to review the charge to reflect the wish of the elected representatives.
Council event facilitation manager Juliette Jones said introduction of fair and consistent event permit fees across the region, coincided with a new Trading & Events in Public Places Bylaw for events, outdoor dining, mobile vendors and markets.
A community category was introduced to cover some costs for community/school events of more than 150 people that had an impact on others' use of the space and facilities.
The school picnic event at Coyle Park was for over 200 people.
However, because it was under 500 people, it was due a discounted community event fee of $90.
A community event of between 501 to 4999 people, would pay $180.
Ms Jones said the council would not charge a school a higher commercial fee if it held a fundraiser.
She said the event permit process ensured safety of all using the park at the time of the event and limited the impact on other users of noise, litter, traffic and general obstructions.