Schools are not taking field trips needed to teach core aspects of the curriculum in subjects like geography - instead having students watch videos in a poor substitute.
"They just can't afford the field trip so they have a video to watch. I don't think that is appropriate and it is not delivering the curriculum in the way it was intended," said Patrick Walsh, John Paul College principal and executive member of the Secondary Principals' Association.
Such examples showed why a major shake-up of how schools are funded needs to funnel more money to those drawing students from the poorest families, Mr Walsh said.
But if the total amount of school funding does not grow to let that happen, high deciles schools with be loath to give up any existing funding, which would lead to school "donations" increasing even more rapidly.
Mr Walsh, who spoke to the Herald from Bangkok where he and 15 other New Zealand schools were attempting to recruit international students, said the higher fees they paid were used by schools to cover core costs, not "nice to haves".
That showed high decile schools did not have spare cash to give up, but Mr Walsh said there were existing "pots" of money that was not accessed.
"That is very fragmented, it is difficult to access, and often doesn't get to where it needs to go. An example of that is the Ministry announcing that they have a whole lot of money available for special needs that hadn't been accessed. That is as disgrace when we have so many special needs students that need support."
The Post Primary Teachers Association (PPTA) holds its annual conference in Wellington from today [Tuesday], and a major topic of conversation will be a wide-ranging review of school funding that has been initiated by Education Minister Hekia Parata.
Ms Parata has signalled that the decile system - which she describes as a "blunt" funding mechanism - will be dropped.
Decile 1 schools are the 10 per cent of schools with the highest proportion of students from low socio-economic communities, whereas decile 10 schools are the 10 per cent with the lowest proportion of such students.
What will replace it, and how radically different it will be, is not clear. A wide range of sector groups have formed the National Education Leaders Partnership (Nelp) in an effort to have a strong voice in another model's development.
The group, which includes the PPTA, NZEI, principals groups and the NZ School Trustees Association, wrote to Education Minister Hekia Parata asking to be directly involved in the process.
Ms Parata responded to say broad engagement would be needed, and her existing Ministerial Cross Sector Forum group would be an ideal place for such discussions.
PPTA president Angela Roberts said there were concerns around how far through the review genuine consultation would take place.
The Investing in Educational Success (IES) initiative, which sees principals and teachers paid to collaborate across "communities" of schools, changed significantly after negotiations with the union, and those should have started earlier, Ms Roberts said.
A spokeswoman for Ms Parata said the sector would be involved in the review, and all peak associations were represented on the National Cross Sector Forum and the additional 40 regional forums.
"As well as many smaller bodies, together with principals, ECE representatives, iwi and Pasifika - there isn't a more representative process and vehicle than the forums.
"In addition the Minister meets with the peak bodies quarterly or on an issue-driven basis. She expects to continue this highly inclusive and consultative approach."