Tricks used by some schools in England to inflate GCSE science results were revealed at a conference on Wednesday.
Some schools switch weaker pupils to BTecs, coursework-based vocational qualifications equivalent to GCSEs, and falsify their marks, claims research.
Author Birendra Singh spent five years observing science teaching in three unnamed London schools.
The Department for Education said any gaming or inflation of results was "completely unacceptable".
Mr Singh, a former science teacher with 17 years' experience, who has also worked as an Ofsted inspector, carried out the research for a doctorate at University of London's Institute of Education.
He told BBC News he had originally been looking at methods of assessment, marking and feedback to pupils.
He observed lessons and carried out interviews with teachers and pupils on an anonymous basis.
"They knew the process would be completely professional, that they could withdraw at any time and that the names of teachers and schools would not be revealed.
"They trusted me. Yes. They volunteered information that I had not been seeking."
He said that questions to teachers on how the BTec marking process worked led to some surprising answers which are outlined in papers to be presented to the British Educational Research Association conference in Brighton.
One report includes conversations with teachers from July this year at a school where pupils judged unlikely to gain C grades in science GCSEs had been switched to BTecs in March, just six weeks before the courses were due to end.
The report suggests that some 110 pupils or 30% of the year group were swapped to BTec at the eleventh hour.
Teachers were asked to help the students complete 18 months of coursework in a fraction of the time and also, it is claimed, in some cases to "invent" or "falsify" grades to ensure they passed the BTec course, worth two good GCSE passes.
The report also suggests that BTec grade manipulation also took place at a second school. Both schools have been rated "good" by Ofsted, adds the report.
In July, Pearson, the exam board that runs the BTec qualification, asked the first school for samples of pupils' work to moderate the marking.
According to the report, teachers were ordered by the deputy head to produce the coursework, leading to a stressful situation just before the end of the school year.
"Nobody had bothered to get these kids to complete coursework - just give them a C," one teacher is quoted as saying.
"Now we are having to get them to complete their coursework. I mean copy, or we tell them what and how to do it."
Another complains of "panic management", saying that some pupils had already left school and were having to be brought in specially to complete the work.
"They are getting nothing out of this. Copy that, copy that, copy that. Then we are marking it and if it gets moderated and we are told it is not right we will have to call the kids in again.
"Effectively I will just slip it through."
Mr Singh said that the third school he studied which is rated "satisfactory" by Ofsted did not enter any pupils for BTec and there was no evidence of grade falsification.
He told BBC News that the rate of cheating suggested in this small study may be indicative "of a bigger picture which needs more focused attention".
A spokesman for the Department for Education said: "We completely agree that this is unacceptable.
"That is exactly why we have tightened rules so that all courses, including vocational qualifications such as Btecs, will be equivalent to no more than one GCSE in the league tables.
"We have also imposed strict criteria to ensure that future performance tables only include qualifications that are externally assessed and of the highest quality.
"Any gaming or inflation of results is completely unacceptable and all schools should have robust procedures in place to identify and stop this."
The proportion of pupils achieving five A*-to-C GCSE grades is key to a school's position in the secondary league tables.
Ofsted chief inspector Sir Michael Wilshaw has said that the league tables create "false incentives" for teachers who try to push as many students as possible to achieve a C grade.
A Pearson spokeswoman described behaviour of this kind as "unacceptable and not in the best interest of students".
"We have a robust system in place to ensure that teachers are upholding the standards of the BTec qualification and we strongly encourage anyone with concerns about malpractice to contact us immediately.
"All BTecs have included a strong element of external assessment since September 2012."
The spokeswoman added that the company supported the government proposals to improve accountability in schools.