MIT (Studious) Model 
The MIT model of Physics education involves creating strong student groups that explore Physics. The actual teaching takes place not in the lecture hall, but late at night in a dorm room around a problem set which is hand graded. The problem set doesn't consist of a large fraction of the grade, and consists of very difficult problems, making it generally impossible for an individual to complete the problems on their own.
There are a lot of subtle things that contribute to this working. It is not in the student's interest to just copy the answers and hand them in, even though this is theoretically possible. It's not in the student's interest, because if they do that, then they will get killed in the test and final examinations which form most of the grade. Also, because the student gets very detailed feedback in the problem sets, it's to the students benefit to know what their weaknesses are.
The other important thing is that the passing grade is set extremely high, and the grades are against some more or less objective standard. Because the pass rate is very high, there is no disincentive to helping someone else learn the material.
Additionally, tests and finals will be hand-graded, with the possibility for generous partial credit. The emphasis is placed on the concept, and not on just getting the answers.