I know that there exists some cases when "have" can be used in the progressive form. But not sure if this usage is permissible in this context: I am at school in the class. My phone rings and I answer:
Do not bother me I am just having a lesson at school.
Best Answer
The verb is OK, but simply "in a lesson" might be better. "having a lesson" makes it sound like you have a private tutor.
just is appropriate when you are talking about something that happened a very short while ago.
If you use just in your sentence, it is not appropriate for the "a very short while ago" meaning, so I would take it as the "only" meaning, which suggests that the lesson is not very important.
If you want to talk about something that is happening at the moment, and is likely to continue, you can say just now