In class homework

I have been using the internet to post the homework assignments for this semester. I have been trying to update the site early in the day. I have noticed that some of the students are beginning the homework in class while I am still going over the section. This happened last year and the students that were doing it were some of my higher achieving kids. This year I have students doing this in class and I looked up a previous grade and they have struggled in math. I am trying to figure out if it is better for me to keep the student involved in class or let the students struggle through it on there own and miss some of what we are doing in class. These students seem to be doing okay so far but what habits are they getting into that will affect their learning later on?