School-leavers still even lack basic skills such as being able to write letters or do simple sums, even those with good exam grades, according to a poll of British employers.
The survey of 26 banks, law and accountancy firms and insurers by the Corporation of London also said many teenage job applicants lacked the confidence to get through interviews.
It has called for schools to give extra coaching in maths and English for business use plus extra in-depth careers' advice.
The study looked at what businesses wanted out of school-leavers with GCSEs and A-levels, who would normally be taken on to do secretarial, clerical, customer support or technical jobs.
Starting salaries in such positions ranged from £10,000 to £20,000 a year, although some school-leavers said they hoped to earn up to £50,000 within a few years.
But many job applicants, particularly those from inner-London, were found to have "raw" social skills.
They had little idea about teamwork, the need for better appearance at work and clearly spoken English.
Many candidates also appeared to be unaware of the jobs available to them in the City, leading to a skills gap in the Square Mile.
One personnel officer surveyed said applicants had to be tested for knowledge of the alphabet before giving them a job that involved filing.
Michael Snyder, chairman of the Corporation of London's policy and resources committee, said: "The success of the financial and related business services sector depends upon the skills of its people.
"City-type businesses need to ensure they can attract and retain the best staff, able to meet the complex skill needs of the 21st Century."