I teach programming, and the large number of in-class exercises I use produces a frequent case of students standing in line to get help from the instructor. One approach I use to address this is having students who have already figured out the answer help the ones who have not. This engages both the good student and the struggling one. There is a down side here in that the good students will sometimes simply give a solution to the struggling student rather than helping them to learn it. I deal with this in two ways: Making this a "class feature" and engaging everyone in the excitement of learning these skills from the beginning of the course, and by rotating through struggling students in subsequent exercise sessions so that I can verify that everyone is learning.