A group of high school principals say some schools have turned a schoolboy rugby competition into an "arms race" by recruiting over-aged, offshore players.

One headmaster says the Canterbury competition is "out of control" due to certain schools actively recruiting 19-year-olds, some from overseas.

Some teams have considered defaulting as a group against certain schools this year due to the issue.

MARTIN HUNTER Will Jordan, of Christchurch Boys' High School, makes a break with the ball during the 1st XV rugby match between Christ's College and Christchurch Boys' in May 2015.

A group of principals were concerned that some schools were taking advantage of the rules to gain an edge, Christchurch Boys' High School headmaster Nic Hill said in a statement.

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The New Zealand Schools Rugby Union (NZSRU) changed the rules last year so 19-year-old non-domestic players could no longer compete

ALDEN WILLIAMS/FAIRFAX NZ Shirley Boys' High School rugby players perform a haka at a competition game last year.

Hill said Canterbury's UC Championship, run by the Canterbury Rugby Football Union (CRFU), was not being consistent with the national rules. It still allowed players who would not be eligible under the national rules to receive "dispensation" to play.

It meant some teams had "actively recruited" older players, some from offshore, for their teams.

"It's my view and the view of other headmasters that the competition is out of control, and becoming too 'professional'. This is not good for the ideals of what we are trying to achieve in school sport," he said.

He refused to say which schools he believed were taking advantage of the rules, but said it was more than one.

It is understood one of the schools in question is Christchurch's St Andrew's College.

In response, the CRFU said the dispensation rule had been in place for 18 years, and had been used by every team in the competition, including the ones now criticising it.

The Canterbury competition comprises schools from Christchurch, wider Canterbury, and the upper South Island.

The situation had led to discussions about an alternative competition, Marlborough Boys' College Principal Wayne Hegarty said.

Other schools to raise concerns included Timaru Boys' High School, Shirley Boys' High School, Lincoln High School, St Bede's College and Nelson College.

The Canterbury West Coast Secondary Schools Association wrote to the CRFU saying there was a gulf between the championship and the "aims and ideals" of the schools.

​Hill said it meant the values and purpose of secondary sport were being lost.

"Over-professionalisation of sport is not helping, it's not helping the game, it's not helping the boys, it's undermining the values we're supposed to be about.

"We're trying to create better kids. The arms race in secondary schools sport is not creating better kids, it's not actually creating better rugby players."

The CRFU said the rule allowing each team to have two under-19 players had been in place for many years.

No teams had broken the rules or acted unethically this year, CRFU chief executive Nathan Godfrey said.

Godfrey had been working with Hill to bring the principals together to address their issues.

St Andrew's College rector Christine Leighton said the school was "supportive of the Canterbury Rugby Football Union process on this matter".