Ellery sent a letter explaining the situation on July 1, which was followed by a three-page explanation from the school board's Kerralie Adam a week later.
Adam said the school would return to having all classes on site as soon as possible.
"Hybrid learning models are a strategy the Ministry of Education supports schools to use when extreme staff shortages mean the school would otherwise have to close."
There had been an average of 10 staff away every day for illness or other legitimate reasons so far in 2022.
"This decision has not been made for convenience, budget or building reasons."
One of the members of the Facebook group, parent Helena Keyes, said the way the move was first communicated to parents and caregivers via Ellery's letter, and the responses given to concerns, "wasn't very good".
Ellery said if the hybrid model was causing unnecessary stress, a decrease in annual leave, or financial hardship to a family, pupils could be sent to school for five days a week.
"We will look after them and they will still do their online component. We keep saying that, and we mean it," she said.
Keyes said that was an option for her family.
She believed it would be hard for teachers to supervise pupils learning at home.
"We are not trying to stir, we are just genuinely concerned about the impact this (hybrid model) is having on our families and community."
"They are worth every dollar we pay them, but we've got to have the relievers to pay the dollars to.
"Then we have to look at if it's a good model to have a reliever in front of a class for a week to two weeks. I don't think it is."
Meanwhile, Whanganui High School will be finishing at 2pm instead of 3.05pm each day until August 10.
School buses will run at their normal times and the school library will remain open until 4.30 pm every day.
Principal Martin McAllen said the school had an excellent team of relief teachers but it was fully stretched, with some unwell themselves.
"This means that our regular teaching staff need to carry out internal teaching cover for their colleagues on top of their own normal teaching load."
Regular staff were also catching Covid-19 or normal winter illnesses and many had their own dependent children to look after as well, McAllen said.
"This means that many of our teachers have to stay at home on a daily basis which further stretches to breaking point our current ability to put a fully registered teacher in front of every single class."
Whanganui High School principal Martin McAllen. Photo / Bevan Conley
McAllen said finishing early ensured every student had a completely competent and professional teacher for every single lesson during each day of the school week.
Schools and kura were doing an outstanding job in challenging circumstances, Ministry of Education director for Te Tai Whenua (Central) Tracey Turner said.
"There is no one solution that will work for all schools and kura, but there are a number of short and longer-term options available that they can explore, including distance and hybrid learning," Turner said.
"We acknowledge this is also a challenging time for students and whānau and not all approaches will suit all whānau all the time.
"We expect schools to work through any concerns with whānau to ensure the education of students is maintained."
Cullinane College Year 12 dean Tony McBride said school hours would be "business as usual" in term 3.
"We've been corresponding with the whānau during the holidays to let them all know where we are at.
"It's been a fantastic start, all the students have come back and look ready to go."
McAllen said he wasn't considering a hybrid model for Whanganui High School.
"Based upon our own personal experience in previous lockdown periods and on recent educational research, nearly all students and teachers prefer face to face, even in the current strongly recommended face mask-wearing environment."
Daily interaction between students and teachers meant the ability to ask questions instantly and get immediate feedback from their teachers or fellow students, McAllen said.
