When I was a young boy, the church we attended had a vibrant men's class. It was a good group of men, and they had enjoyable class sessions, but the discussion was often on anything besides the lesson. The teacher would start the class off with scripture, but the culture of this group was such that one topic would dissolve into another, and that one into yet another. These men might "solve" the problems of the world, but never seriously consider the assigned scripture.
It might be too much to ask for a class never to get off on a tangent. In fact, tangents might be helpful at times. But if this happens too much, the integrity of the teaching moment can be lost. Our lessons are designed with a specific spiritual truth and objective. At the end of your lesson, do your students know both the truth and how to apply it in their lives?