Damon Martin-Bailey, who lives nearby, said he heard a "small bang" on the road, followed soon after by sirens.
When he went out to the road, he saw a person lying under a blanket on the crossing.
The girl was taken to Waikato Hospital. Police initially said her injuries were critical.
A Waikato Hospital spokeswoman said she was still under observation in ED, and was now in a serious condition.
Waikato Road Policing Manager Marcus Lynam said police had spoken to a number of witnesses including the male driver of the ute.
He appealed for any other witnesses to contact police.
"Police also remind all motorists in general to take extra care when driving
near school zones," he said.
The school confirmed that the girl was a student, but could not comment further.
In a post on Facebook, a staff member said the student was "in the best possible hands" at the hospital.
"We would like to send our sincere thanks to the Hillcrest community members who were present at the accident and provided exceptional support.
"The police have also asked me to acknowledge and thank the community for responding appropriately and for making a helpful contribution to the management of the scene.
"Our thoughts are with everyone involved at this challenging time."