Although lazy is often used, in academia there are a wider variety of words that can be used with less negative connotations.
We can say that a student is unengaged, unfocussed or detached. We avoid lazy because sometimes the fault is not always with the student but due to other circumstances either inside the class or elsewhere. It could result from sickness or inability to understand the teacher (such as being weak at understanding a second language etc.)
The aim of a good teacher is to get all their students engaged and focussed and overcome their difficulties.