Every bell curve has a front tail and a back tail. These "tails" are the portions of the class that score at the very high or very low end. The back tail represents those students whose scores fall far below the average. One problem with bell curves is that students at the back end are ignored, because teachers focus primarily on improving the average, represented by the peak of the bell curve. In focusing on the middle majority of students, teachers may fail to address the needs of students struggling with the material.
The front end of a bell curve is also frequently ignored by teachers using a curved grading scale. It is almost easier to ignore students at the front tail of the curve than students at the back tail, because these students do not appear to need extra attention to grasp the course material. However, by ignoring these students, teachers are missing out on an opportunity to encourage their more gifted students to make the most of their aptitude, by pushing them harder.
One problem with the practical grading element of a bell curve is that there will rarely be a perfectly normal grading distribution. In other words, some students may be disproportionately above or below the average. This has the statistical effect of distorting the grading distribution. For example -- if, on a particularly difficult test -- almost every student scores between 20 and 25 percent, but one student scores a 98 percent, the "average" may be much higher than 20 to 25 percent, which could result in much lower grades for the rest of the class.
When using a bell curve to evaluate students, the students are being scored relative to one another. If all of the students are struggling to comprehend material, a curve will ensure that the grades are not lower than would be fair, given the difficultly faced by all of the students. At the same time, a bell curve can mask deficiencies in a teacher. For example, if a teacher is using a curve -- and students are scoring between 15 and 30 percent on assignments -- some students will still receive A's and B's. However, even though some students are receiving high grades, they may not actually be learning the material sufficiently.