Overachievers should not be able to raise the bar. If they can, then the system is broken. Grades should be relative to the course content, not the abilities of the students.

When I started university, it was clear that I had much more experience programming than the other students, but I had to do certain introductory classes anyway because there was no way to receive credit for half a decade of self learning. On one assignment my lecturer went looking for things to mark me down on that were outside the scope of the assignment because "it would look suspicious if you got a perfect grade".

Some people are just capable of more than is expected of that class for whatever reason, not because they intend to demoralise everyone else and break the system.