Mina, be careful to over simplify the issue. For some students if you allow them to speak with you about the subject matter you will realize they are well prepared and know the material well. However when you issue them an assessment their score will not reflect their knowledge. Any number of issues can account for this issue including learning disorders and different types of anxiety. If you encounter any students where this is the case you will want to be prepared to discuss some alternate methods of assessing their knowledge of your lesson plans. James Jackson