If a student has joined the wrong group, you may want to remove them from your group.
To remove students:
Click the group name (#1 in the screenshot below).
Click "Group Settings" (#2 below).
Click the names of any students you want to remove from the group (#3).
Click "Remove" (#4):
Your student(s) will be removed from the group, and your conversation with them will be deleted.
Note: if you want to remove a conversation with a student, but don't want to remove a student from the group, you can leave the conversation instead.