Tally the students' scores and have them all easily available. To create a bell curve of the scores, you’ll need to have data for each student. Even in a class of 500, you need to have an accurate score for every student before you begin constructing the bell curve.

Find the arithmetical mean and standard deviation of all the students’ test scores. You can do this using a computer program, a graphing calculator or simply by hand. To calculate standard deviation, subtract the mean from each test score to find each score's deviation. Square each deviation, then add up all the squared deviations. Divide that sum by one less than the total number of exam scores. Take the square root of that number to find the standard deviation.

Make the mean test score a C grade, regardless of the actual percentage of the mean score. That score is now the cutoff for a C. Add the standard deviation to the mean score to get the cutoff for a B grade, and add one more standard deviation for an A grade. Subtract one standard deviation from the mean to get a D grade and subtract one more to get an F grade. You can add and subtract half a standard deviation from these grades if you want to assign “plus” and “minus” grades on top of straight letter grades.

Convert the raw test scores of your students into curved scores based on your determined cutoffs. If the previous average score was 60 percent, a student who scored 60 percent on that exam should be recorded as having earned a C. Do this for every score, and you will have successfully graded on a bell curve.