Nearly 2000 students and staff were sent home early after a bomb threat at a Hamilton high school on Thursday morning.

Hillcrest High School was evacuated at 10.50am after a threat was found written on the wall of a toilet cubicle.

The threat said a bomb would go off in classroom E7 at 2.20pm, a Hillcrest High School teacher told students.

CHRISTEL YARDLEY/FAIRFAX NZ A threat written on the wall of a toilet cubicle was credible, said Hillcrest High principal Kelvin Whiting, pictured in the orange vest.

A police bomb squad gave the all clear at 4.30pm after a building search.

All students had left safely by 4pm, after spending hours on the far side of the school field.

Principal Kelvin Whiting said the threat, written on the wall of a toilet cubicle, was found by a student.

CHRISTEL YARDLEY/FAIRFAX NZ A specialist bomb search team was brought in.

"This happened fairly quickly, really, us being notified," Whiting said.

Text alerts and emails went out to parents and the school updated the students based on police advice.

A bomb squad from Auckland arrived an hour or so after the evacuation and a Hillcrest student said it took nearly an hour to search just one room.

CHRISTEL YARDLEY/FAIRFAX NZ Students were kept on the far side of the school field while the bomb squad searched the school.

"That's why they decided to have a 'controlled leave', because they didn't know how long it would take to complete the search.

"Cars parked in certain areas were not allowed to be moved and a lot of school bags are locked in classrooms overnight."

Students were told to leave by Carrington Road.

Senior students were allowed to walk home from 1.30pm, but those under 16 had to wait for a parent or the regular schoolbus.

According to a student at the school, a helicopter containing a bomb squad arrived on the scene just after noon and a fire truck and ambulance could also be seen.

John Twizell​ went to the school after he got a text from his daughter, a Year 10 student, and couldn't find out what was going on.

"The best way to get information is to come right down here," he said.

"It's probably a false alarm... But there consequences of ignoring a potential threat like this could be enormous."

He didn't get the school's alerts but said he had no major concerns after hearing all the kids were safe on the field.

Another parent said her daughter phoned to tell her not to come to school, as they had been evacuated due to a bomb threat.

"The school is taking it seriously and the bomb squad has been called in," the parent said.

Parents were warned to stay away an initial text from the school, which said "HHS currently evacuated at 10.50am, 27Oct, Office unattended. Do NOT phone or come in."