But at the same time, they'll usually tell you up front that they're curving based on other students. If you've established a certain rubric on which you're grading your students, they're commiting the the class. In most universities, once the class is half over, you don't have an opportunity to drop the class if you don't agree to the new grading criteria.
True with med school classes -- but in most of MY classes (non-med school) the teachers do as they please so we just do the best we can do no excuses. Regardless how the teacher makes the syllabus you get the same grade whether it was changed half-way through or the final version was presented first. So I say just do the best you can do no matter what (even if you have a 100% you just never know).
Also, we can withdraw from a class up to the last day, up to the day before finals.
If its really a GPA killer you should probably talk it over with the professor, but I think the grading scheme is always able to change because it applies to the whole class and doesn't negatively affect 1 student.