What is an eighth grade students intelligence level?

Here is my guide:

• A+= has a very powerful strength in a subject
• A= has a powerful strength in a subject
• B= has a strength in a subject
• C= has a somewhat strength, room for improvement
• D= has little strength, needs improvement
• F= has very little or no strength in subject.

Sometimes bad grades are do to not very well study habits or even nervousness.