I have had experience with colleagues welcoming inexplicable improvement for which they will get credit, and I believe teachers have a responsibility to discourage cheating . Certainly as the stakes get higher, it becomes harder to disguise, so better to stop now. Address your concerns directly with the student in the presence of a personal tutor and act accordingly. You should ask to see notes and sources used to support coursework. If these are not forthcoming, you might be right. Importantly, get to the root of this deception. Is the pupil struggling? Are there personal problems? Don't just assume she is cheating for the sake of it.