The whole point of teaching by the end of a lesson is that students should have learned whatever it is they were being taught. Effective teaching demands that students should, by the end of the lesson, have understood and can apply what they've learned. While most teachers make lesson plans with well-defined objectives, often, many fail to accomplish the goals they set. I believe that though time constraints might make it impossible to teach the same thing over and over until everyone in class has "learned" what they were being taught, one should try to get other students involved in the process. One teacher I know encourages students to form discussion groups. The students who learn faster get to help those who are a little bit slower. This seems to work well enough because students are bound to be more open with their peers, admit they haven't understood something. Using reports provided by group leaders, the teacher will use that information to determine whether they should change strategies to ensure students actually understand what they are being taught. Your thoughts on group discussions as a way to get students to learn whatever they may not have understood?