More than 15,000 students are estimated to have signed up to one website, which charges pound;9.99 for access to nearly 40,000 assignments. Those who prefer to pay in kind can submit three essays instead. The site is one of at least six in the UK offering completed coursework or tailor-made essays.
This week, with A-levels and GCSEs about to begin, there were fresh calls for the Government to investigate the sites.
David Hart, general secretary of the National Association of Head Teachers, said: "These websites encourage cheating. The great danger is that coursework will be undermined, if it can be so blatantly subverted in this way."
Exam board Edexel faces controversy for providing a detailed 12-page model answer to one of its maths GCSEs in a guide for teachers available on its website.
One student told The TES: "It would be easy to pass this work off as your own. In maths, unlike English, it is difficult for teachers to tell if the work is original or not."
Last year, examiners criticised teachers for signing off coursework as original even when it was obvious that pupils had colluded. And there are fresh suggestions that some teachers are bending coursework rules.
Contributors to TES online discussion groups say that teachers, under pressure to improve results, routinely push the boundaries of coursework rules. One science teacher said: "The simple truth is that with a lot of care on the teacher's part, nearly anyone can gain an A at GCSE."
Another said: "I have seen teachers plot graphs for students, annotate work not with questions but with answers, and tell students exactly what they should be doing."
A teacher of General National Vocational Qualifications courses said: "I see pupils copying whole pages from textbooks and then portfolios marked as if they were the work of the pupil."
All of the major websites have statements condemning plagiarism.
Coursework.info, the largest site, said its essays could not be cut-and-pasted into other documents. Charlie Delingpole, company director, said: "There's a valid educational use for these sites. Students use them for revision."
However, there are fears that the system is being abused.
Coursework.info claims to have 49,000 registered users, at least a third of whom are of school age. It said many members were teachers wanting to check their students' work. It offers 27,730 GCSE essays, 8,949 for A-level and 970 for International Baccalaureate.
A spokesman for the Qualifications and Curriculum Authority said there were concerns about the sites. But he added that there were strict guidelines for boards to crack down on cheating.
But some teachers are unconvinced. Alastair Cornish, head of maths at a Cardiff secondary, said: "These sites make a mockery of coursework. In maths, there is a definitive answer for most questions and it is very difficult to tell if someone has used a website. You are likely to give them full marks."