The article from the DomPost (also in Stuff) starts with:

Schools have been told they can now search pupils for weapons and drugs without fearing legal action.

This is incorrect and the quoted advice from the Secondary Principals' Association's lawer does not indicate this at all. Rather it says that teachers can ask pupils to empty their pockets or open their bag/locker. If the pupil refuses to do so, the only option is to call their parents/guardian and ask them to come in.

There is no right for the teachers to force students to be searched and the Secondary Principals' Association's legal advice does not indicate that there is.