Strange question, because it gives the answer: academic achievement vs. class the student is enrolled in.

I try to clarify, read the following sentence:

The teachers analyzed the results according to the grade of the students.

As a reader it is difficult to know if the teachers analyzed according to achademic achievement or class. How to distinguish this?

A possible substitute for grade in the achievement sense is "mark" (more common, I think, in British usage than American). "Academic marks" might make this even clearer. For the other sense, you might consider "grade-level" or perhaps "year in school."

Also, you can add a statement early in your write-up to stipulate how you will be using the terms, and define fully them.

Then your sentence could be either "The teachers analyzed the results according to the academic marks of the students" if you mean to refer to academic performance; or "... according to the grade-level of the students" if you mean to refer to class.

oops, I meant *fully define them*, not define fully them.

Also, I think in British usage "form" corresponds to what I called grade-level or year in school. So that might give you another option.

Thank you, would it be wrong to write achademic grade since I'm writing in american english?

I think it would be better to use "academic grades" (plural). A student gets a grade, or a mark (singular) on one specific examination or in one particular class. But the grades or marks (plural) that a student gets reflect achievement in all classes taken in a given time period.

Using the plural term for achievement would also help emphasize the distinction between that and grade-level (singular). Along with stipulating the two different definitions early in your write-up, that should avoid confusion.

Thanks for your help!