The Common European Framework for Reference Languages (CEFR) is a defacto standard that:

... describes in a comprehensive way what language learners have to learn to do in order to use a language for communication and what knowledge and skills they have to develop so as to be able to act effectively.

Use of the CEFR

The reference framework covers many languages, including English. It is used by growMe4IT as the basis for checking and directing student's English language proficiency.