The goal shouldn't be “Get a higher grade than my classmates”, it should be “Get the highest grade possible”. And the goal for the teacher should be “Translate each student knowledge data into a numeric value”, which is somewhat orthogonal to the experiment.
Any college student with the sole goal of learning how to learn & think unfortunately doesn't survive the education system.
During the course of a class, if as much education is given to each individual possible, then at the end of the class what does the actual grade matter? These grades turn out to be a poor proxy for things like earning, ingenuity, creativity, changing-the-world, or for that matter even grokking the materials. The only thing for which they're (maybe) a valuable proxy is 'how well can this student juice the grade of the next course in the line' -- a question whose answer ultimately adds no value to society.
It's unfortunate, but in my department about 80 % of the students go up to med school. The GPA plays a great part in determining just what school will accept you, which in turn has massive impact on your future earnings.
The other problem with this approach to "group learning" is that it doesn't take into account freeloading students, i.e. those students that want to do the minimal amount of work for the best possible grade. Anyone that did a group project in college or high school knows that there is always that person who just doesn't care, and is happy to get whatever the group gets.
But in voluntary schools where you pursue your own interests (and not some certification), grades are so ineffectual that they're generally nonexistent. At least in my experience. Tests may corrected, but there isn't a grade. You just see what you did wrong. When you ask the teacher for more detailed feedback on your progress, they don't pull a number out of their butts, like "2.7". That whole grade infrastructure is for another reason.
As for "freeloading students," no one cares. (In these more voluntary forms of education.) It is your choice how much effort you deem appropriate.
Another goal is "get the best possible grade", which some students might not have, but most do, and different students value it differently. But depending on the class, this MIGHT reduce to "get a higher grade than my classmates". Or it might not. If you have a chance (and willingness) to affect your classmates' grades, then knowing whether or not the class is graded on a curve is pretty important.
I love it, I think this pretty well sums up how much certainty there is about what people want out of education :)
Which isn't at all easy. Grades certainly have some correlation to knowledge (certainly not "one" though), but in a perfect world we'd be putting everyone through oral examination and putting their projects through peer review on a constant basis. This is expensive though, in time and money and man power.