Station officer Andy Hamberger, of Kerikeri Fire Brigade, says flames were shooting out the toilet doors when he arrived.
Kerikeri High School is due to reopen to all students on Monday after a fire destroyed a toilet block and damaged three classrooms.
The blaze, which started about 2.20pm on Wednesday, is still being investigated.
All 1500 students stayed home on Thursday on the advice of the Ministry of Educationwhile a thorough damage assessment was carried out.
Senior students, in Years 11-13, went back on Friday.
In an email to parents principal Elizabeth Forgie said seniors had come back early because it was close to the end of their school year when every day counted.
The final site investigation and insurance checks had taken place on Thursday afternoon.
The impact was worse than initially expected with three classrooms out of action for some time, with significant effects on lesson timetables.
Forgie commended staff and students for their calm and patient evacuation and she was ''hugely grateful'' to Kerikeri Fire Brigade for saving the neighbouring business, enterprise and social sciences building with 10 classrooms, a learning common and resource rooms.
Firefighters hose down the back of the toilet block during Wednesday's blaze at Kerikeri High School. Photo / Peter de Graaf
Flames from the toilet block's rear windows had jumped a walkway and spread to the building, but quick action by the brigade limited the damage.
Sergeant Phil Le Comte, of Kerikeri police, said investigations into the cause of the fire by police and Fire and Emergency NZ were continuing.
Anyone with information about the blaze was urged to call police on 105, Crimestoppers on 0800 555 111, or the school's associate principal Mike Clent.
