There was no one near the horse so police called animal control.
A Rotorua Daily Post reporter at the scene said he later saw a man get on the horse and ride off within 15 minutes of the police being called.
Rotorua Intermediate principal Garry de Thierry said he first saw the horse outside the school at 7am.
"It surprised me to see a horse all saddled up in the middle of suburbia.
"The horse seemed pretty passive."
He said students stopped and pet the horse.
"It's something quite new to them as well and out of the ordinary."
The New Zealand Transport Agency website said horse riders had to follow road rules including riding on the left-hand side of the road, and wearing a safety hat.