"I have heard of things like shade sails being damaged and there have been burglaries at a couple of schools recently but I have never come across anything as sickening or mindless as this," Mr Hantz said.
He suspected the intruders had got in through a door which possibly had an air-lock problem and may not have self-locked properly: "There is no other way to explain how they got in so I suspect that could have been it."
He said the culprits would have been on site for little more than five minutes because that was how long it took the security response officer to get to the school.
The officer discovered the mess and called a staff member who lived near the Church Rd school.
"All we could do was lock it up and deal with it in the morning."
The school's caretaker and cleaning staff went to "the nth degree" in cleaning up, Mr Hantz said.
"It was absolutely clinical - it had to be."
With the classroom closed off and police at the scene first thing Tuesday, children were told at assembly that "bad guys" had broken in and thrown mud everywhere and it had to be cleaned off.
But there was more to come. Staff also discovered that on their way from the grounds the intruders had used scissors taken from the classroom to cut and shred a sports net for soccer and cricket which had been up only for about a month.
"It was ruined - ripped apart," Mr Hantz said.
"It can't be fixed so we will have to look to replace it."
He said staff and parents were devastated by what had happened, with one parent posting a message on the school Facebook saying "what a sad and horrible way to end the school year".
Anyone who may have seen any suspicious activity around the school, or has any information about who may have been responsible, is asked to contact Napier Police.