Is it possible to reset a student’s activities in a lesson (Course B)?
If you are referring to a single lesson, yes. There should be a start over button above the code blocks. (See image below).
If you are trying to reset the entire course, no, there is not currently a way to do that, but I know it is a popularly requested feature and is something code.org is looking into implementing at some point in the future (no official timeline yet).