Como dizer "Nivelamento escolar" em inglês

One more context I thought of, is that of "assessing their skills" to track special difficulties. Them level them up (the group of students I mean, not the difficulties)
This would be done, for instance, after coming from holidays when some students get somehow rusty!

Uma tradução direta e usual:
School "levelling up" ou Education "levelling up".

*Sugiro usar "levelling up" entre parênteses, pois trata-se de uma espécie de "substantivação" do verbo frasal "level up" (subir e igualar, nivelar posição).

I would say that you have to strike "a balance between different levels of student" teaching.
That is, you have to use a teaching method that equalise between the levels of earning of the students.
Could also be taking an "adjustment approach" to (teaching) students with different skill levels, levels of learning, etc.
But then, these are not academy translations, just a layman description, from my point of view.