Does it strike you as a good idea to judge teachers by the exam results their pupils achieve, then give them a piece of work weeks in advance that their pupils must carry out under 'exam conditions', then tell them to mark the papers? Should we be surprised that these coursework marks are often very high? It's a bit like telling driving instructors that they will only be paid if everyone passes.
Coursework used to be given to the kids to take home and obviously those with tutors or nice bright parents would gets tonnes of help. Now it is generally done in school so that we can make sure that all the results are fiddled equally. Cheating is absolutely endemic for the obvious reason that it isn't in anybody's interest not to. Dubious practices range from writing the answers on the board to 'just happening to teach something very similar the day before.' The temptation to read meaning into some vague answer and mark it correct is very strong indeed when you know that Mrs Wade down the corridor is doing the same and you might well be compared with her.