This is the way I understand it, but I might be wrong. Anyway, this is my interpretation :

"in class" refers to the location = The students are in class/at school now (in a classroom following a physics class/course for example...)

"During  class" refers to the activities that are going on in the classroom = During the lesson

Young boys often have trouble focusing in class

Young boys often have trouble focusing during class

Both are grammatically correct, and in this example one can use either one or the other.

Yes you can.  "In class" however can also mean in attendance of class.

For example:

A college student might get some sort of text message from a friend:

"Hey bud, we're going to the movies, wanna come?"

"Nah man, I'm in class."

You would not say, "Nah man, I'm during class."

I hope this helps