At the beginning of a term, I'd always get at least one teacher that says "you all have As now, and you can keep that as long as you don't mess up." (It's usually worded a bit nicer, but that's the idea.)
I HATE that mentality.
It means, to keep an A, I have to be on the top of my game for every test, every assignment, and every class. It means that if I screw up and stay up late before one test, I might mess up my grade for the rest of the semester. It means that one night can ruin my perfect 4.0 (which I gave up on after freshman year), thus "ruining" my chances of getting into my college of choice.
Luckily, I decided after freshman year that the difference between a 3.8 and a 4.0 was not worth the huge amount of extra diligence it would require. And that has paid off: instead of wasting time on HW and studying, I've spent my time learning Python and C and Linux administration and dozens of other potentially useful skills (that haven't helped much in college applications, but will be useful in and after college).
EDIT: Something else I wanted to add to this:
I've thought about this a bit, and I think one of the big problems is how our grading system is structured. Like I said: if you mess up, you're (in theory) done.
My ideal system would be the opposite (mostly). At the beginning of (high) school, a student starts with 0 in each subject. From there, school would be similar to an RPG: classes would have a prerequisite "level" required, and each class would gain points toward "leveling up." In each class, each assignment would be worth a number of points, and the total points earned at the end of the term would determine the points earned for the course. In order to graduate, a student would need certain "levels" in each subject.
This would not punish nearly as much, allow the best students to progress faster, and allow slower students to take it more slowly.
I haven't written this down before now, so excuse any glaring holes it may have.