If you'd like to reorganize your students, for example moving some of them from one class to another, these are the steps you should follow:
1) Go to "My Classes" select from the left bar the original class that your students belong to.
2) Go to "Members"
3) Remove the students we want to change to another class
4) Go to the new class and click the "Invite students" blue button at the right.
5) Then give the new class code to the students you want to add to that class.
Here's a quick video that might help.