I am at the lesson./ I am in the lesson.

[englishhobby]

Is the following exchange natural?

A mother is calling her daughter who is at school at the moment:
Mother: Where are you?
Daughter: I'm at the lesson.

Must we use in?

 

[BobK]

'I'm in a lesson' or (if it's one they've discussed before)'I'm still in the lesson'.

b