The lack of response means "no"
Sorry, I don't know how to do it. Generally to do something like that will mean a tweak to the lesson module code. That would impact the whole site. I was thinking of maybe using a course theme that would display the student scores white on white. However, Lesson uses a class called a generalbox, so that is not going to help with a theme, without also adjusting the lesson php code to create a new class for that box. And that is going down the rabbit hole for a hack.
You could make it a suggestion as a Lesson improvement in Tracker. Probably not that big a deal to add that as a Lesson setting. Not all teachers want to tell their students their score (this is a hard concept to grasp for some) at the end of an activity, so it is a good suggestion.
Somebody with more talent than me will have to step in incase the answer is "yes and here is how", or 'no but have you tried".
Sorry I could not be of more help. Best Chris