College managers have admitted that they recruit students on to unsuitable courses because of the pressure to fill places.
Schools and colleges are fighting over A-level students, leaving less able pupils to fend for themselves or drop out of the education system entirely, according to a study of post-16 provision in London.
Researchers at London's King's College School of Education have followed the progress of comprehensive pupils from age 15 to 19.
Their study reveals the stark choices that face some 16-year-olds. Top performers take their pick of institutions while underachievers are poorly catered for and often fail to fit the narrow categories left open to them.
Students, parents and staff at 14 post-16 London schools, colleges and training and enterprise council-backed schemes were interviewed.
Professor Stephen Ball, the report's co-author, said the the institutions were driven by the "economics of student worth" in which A-level students were the most highly-prized catch.
"Within this economy some students are much more valuable than others," he told The TES. "And everyone wants A-level students because they stay on course, they are cheap to teach and well-motivated - you don't have to chase them to attend or spend time keeping them on the course - and they are good for the reputation of the institutions."
Female students are also much sought-after, his report says. One provider commented: "If you get the impression it is a college that looks as if it has lots of females and treats them well, you will get lots of women - and that will always attract the males anyway."
The pressure to drive down unit costs to meet Further Education Funding Council spending criteria means colleges are falling over each other in the scramble to sign up 16-year-olds, most noticeably in parts of London, where costs are highest and local competition most intense.
"Particularly in this area, there are very high levels of competition and an over-supply of places," Professor Ball said. "They will do anything to attract students."
College managers freely admitted placing students on unsuitable courses to fill quotas. Others spoke openly - during anonymous interviews - of their pleasure at hearing of drugs or discipline problems at neighbouring institutions.
"The old co-operation which once existed between providers in our locale has disintegrated," the report concludes.
Disinformation was rife, Professor Ball said, and not only among competing colleges. "This is also a problem within the 11-18 school sector. They want to retain as many students as they can at 16 so they don't have much interest in telling them about other opportunities."
The research identifies five types of student. The "embedded" students were motivated and career-minded, often middle class, well-informed about best opportunities.
The largest group are described as "pragmatic acceptors". They saw education as a means to an end - a job. A third group "potential acceptors" recognised the need to stay in education but were unenthusiastic. They believed "simply being present in the school or college it is somehow possible to become `educated' even when playing cards in the common room".
A fourth group, "hangers in" wanted a fresh start at 16 after an unhappy school career. They enrolled on low-level vocational courses at college but had trouble settling in and making friends. They saw college as a place to be until something better came along.
A significant minority were "outsiders" with poor qualifications. They saw further education as "more of the same" and soon dropped out.
"Choice is a slippery concept," says Professor Ball. "Some of these students can potentially choose any route but others can `choose' only one route." Such pupils had no desire to resume their education and were beyond the reach of lifelong learning initiatives.
"The Government tends to talk about all students as if they were the same in the market-place but they are not. Some of them had had enough of education. As one of them said `we don't want no more learning'."
Colleges' increasingly high-profile advertising campaigns appeared to have little effect on potential recruits and were even viewed with scepticism.
"Colleges spend tens of thousands of pounds generating market information - it's a massive expenditure that's fairly unproductive. Basically it was word of mouth that was the key factor," said Professor Ball.