More than 400 teenagers were removed from Birmingham schools just months before they were due to sit GCSE exams, helping boost the city's pass rate.
A total of 427 youngsters - 3.6 per cent of the total due to sit last summer's tests - were taken off the roll at 63 secondaries between September 2004 and January 2005.
Education scrutiny officials last night said the figures raised "massive questions" about how schools dealt with difficult pupils.
Concern was also expressed after the authority admitted it had no idea where 111 of the pupils ended up.
An inquiry was launched by officials after The Birmingham Post highlighted last November the large number of pupils taken off roll at Kings Norton High and The College High.
The College High, in Erdington, took 56 Year 11 pupils off roll and Kings Norton High removed 32. Both were later praised by education bosses for tripling GCSE pass rates.
Councillor Jon Hunt (Lib Dem Perry Barr), chairman of Birmingham's education scrutiny committee, said: "These figures have never been collated before for the city and as such we have no comparative figures available.
"I am concerned to get confirmation that the LEA loses track of some pupils and want to ensure that this does not happen in the future.
"This requires the need for robust and close monitoring systems."
Of the 427 pupils - out of 12,342 due to have sat the exams - taken off roll, 201 were referred to alternative educational provision, mainly with vocational training providers, colleges and behaviour support units.
Most of these - 57 per cent - failed to gain any qualification at all.
Of the others, 81 moved out of the city to other authorities or countries; 17 refused to attend school; six were at independent schools and three had long-term illness.
Eight were listed as being in custody. Out of the 'missing' 111, 32 were not even reported to the education welfare officers.
Bill Anderson, deputy general secretary of the Birmingham branch of the National Union of Teachers and education scrutiny committee member, said: "Clearly we need to have a look at what is going on behind the scenes.
"This raises some very uncomfortable questions for schools. With pupils referred on to other providers, if that provision is not such that it is going to give a qualification to the student, you have to ask whether it is a proper provision."
No one from Kings Norton High or The College High was available for comment last night but a breakdown of pupils removed from The College High shows 35 ended up in alternative provision and the location of 16 was unknown. Two were said to have moved to other parts of the UK, two were in custody and one was "refusing to attend".
The school boosted its pass rate of pupils gaining five or more A* to C at GCSE from 12 per cent in 2003/04 to 34 per cent last year.
At Kings Norton High, 20 were referred elsewhere; the whereabouts of six was not known; three had moved to elsewhere in the UK and three refused to attend.
The GCSE pass rate at Kings Norton High rose from 16 per cent to 50 per cent.
The impact of taking pupils off roll on the city overall will be highlighted in a report due to be submitted to Birmingham's education scrutiny committee next Wednesday.
It shows if they were included in the results the proportion of pupils hitting the benchmark target would have slid from 56.7 per cent to 55.2 per cent. The national average pass rate last year was 57.1 per cent.
The report concludes there is a need for "reinforcing guidance on the process for taking pupils off roll".
It also calls for further guidance on alternative provision for teenagers.