If you want to keep your students assigned to a group but you want a specific group's games not to be available to students, you can go to the group on the My Groups page and uncheck the Visible to Students checkbox as shown in the picture below. https://www.pinkcatgames.com/mygroups
Or you can un-assign students from groups in the Student List Editor. Just click on the group dropdown for each student and click on the group that you want to remove them from.