Lydia Naera, who lives opposite the school gates on Clayton Rd, said she first noticed emergency services at the school at about 11.45pm.
She said she was driving home from the pokies when "zoom, an ambulance went straight past me".
"I thought 'I will go a little bit faster and see where the ambulance is going,'" she said.
"I came into my driveway and I parked the car here and I got out and I walked back too. That's when I saw the commotion over there ... Where the ambulance was, it was in there, in the field."
She said she saw the ambulance "scraping itself over the bumpers" to get to the middle of the school, and the "police kept shooting past".
"All the cop cars amalgamated out here all the cop cars and I'm like something bad's happening!"
She watched them from her bedroom window for half an hour before going to sleep.
"They were there for ages," she said.
"All I could see was floodlights and police and ambulances all in that area. I could see people running like cops."
She said there were four or five police cars outside the gates by her house, and more inside the school with two ambulances.
"It's lucky it's not school time otherwise the kids wouldn't be able to get into school ... I have never seen nothing like that."
In a Facebook post this morning, St Michael's principal Kristina Crouch said the Board of Trustees was both shocked and saddened at the news.
"The police have taken control of the school grounds and we are awaiting further information from them," the post said.
"Our school is a close and tight-knit Christian community, and we know that all of our staff, students and their families will be as shocked and upset about this as we are.
"Obviously, our first concern is the wellbeing of our students and their families. We would also like to extend our deepest sympathies to the families involved. We encourage our faith community to offer their prayers to those who grieve."
Crouch said in the post that more information would be forthcoming in regards to school on Monday as the school work alongside police.
Police would also like to hear from anyone else who may have heard or seen anything in the area - for example, yelling or arguing – between 8pm and 11.40pm.
If you can help, please call Rotorua police on (07) 349 9400. Information can also be provided anonymously via Crimestoppers on 0800 555 111.