• Overcrowding and poor facilities, including up to 64 students in some Auckland classes.
The External Evaluation and Review (EER) says interviews with staff and students and school documents also identified "very high plagiarism rates" of between 20 and 50 per cent. Yet the even higher rate of plagiarism in the assignments had not been identified by Linguis staff.
"The scale of this plagiarism brings into question the reliability of the reported figures on educational achievement, and the processes leading to them."
It says some Linguis teachers seemed unclear about what counted as plagiarism, which would make it difficult for the school to tackle the problem.
Reviewers also saw some low-achieving students who had been at the school for months and heard that many others could not succeed because of their poor English. Students often submitted work late or repeated tests they had already done.
It listed a catalogue of failures to meet standards, including not reporting credits for a large number of students and having to pay a shortfall to the Public Trust, which holds student fees.
The majority of the school's marking assessments were overturned by external moderators three years running from 2011 to 2013.
In May last year NZQA ordered Linguis to stop offering places to Indian students unless they had achieved an internationally recognised English test in the past two years.
The report says the school, which has two sites in Auckland and one in Christchurch, grew rapidly from only 158 students in 2012. About three quarters of its 1000 students in 2014 were at the Auckland campuses, 95 per cent of them Indian.
The report notes that the owners also operate an immigration agency, which provides immigration and employment services.
Publication of the report was delayed by a legal challenge from Linguis, which sought a judicial review of its findings.
Linguis director Mike Dawson said the school first encountered the high plagiarism levels when a large number of new students arrived after the Government relaxed English testing in 2013.
"Linguis immediately set about addressing this plagiarism and voluntarily, honestly and openly raised it to NZQA when scoping the content for our upcoming 2014 EER.
"Linguis does not regret that it raised the issue with NZQA at such an early stage, but naturally is disappointed with such a poor EER result."
Dawson said the school now had less than 200 students. He confirmed it was among a group of 18 education providers with Indian student visa problems called in for a "please explain" meeting at Immigration NZ's Auckland office on October 31.
NZQA deputy director Grant Klinkum said it was unusual to find such a high level of plagiarism, which was a key factor in giving the school a "Not Confident" rating.
The series
• Monday: Visa and school fraud
• Tuesday: Student exploitation
• Wednesday: Effect on immigration