Draw a diagram of a bell to detail the curved shape. On one side of the bell write "highest grades," over the curve write "most grades" and on the opposite side write "lowest grades." Refer to this diagram, but avoid words like "good," "bad," "best," "worst," "pass" or "fail." Indicate how curves in grades, as in those on bells, are natural; some students will succeed in some areas better than in others. Where they end up on the curve is not a criticism, just a statement of fact in regards to this one issue.
Reassure students that grading in this way is not a punishment. Students who scored well will not have their grades brought down by other students. Also, student who scored poorly are not hurting anyone; they just struggled in this area.
Do not identify any students by name. This can make students and their parents feel uncomfortable. Elementary school students are new to this concept and often abstract concepts as a whole. Attaching actual names to your lesson may cause them to internalize the meaning of this bell curve too much and may shape the way they think about themselves and others as students.