Defintion is important, so can you use other terms that I can understand( Intermediate etc), or explain the context? Do you mean 3rd grade in a school? Age of students?

Ability in oral English, or one of the other three skills? All four skills?

Personally I leave the diagnostics until I get to know a little about the students in class. This could be during the first week.

As this in an academic assignment you can set the parameters yourself.

I've changed the subject heading so it is clear to other posters, who may also offer help if you are specific.