It is not uncommon for students to transfer out of a school during the school year.
In ClassroomParent, it is easy to remove that student from the system so that they and their family no longer appear in the directory, nor have access to it.
Navigate to the students homeroom and hover your mouse over the line in which the student is listed.
Two icons will appear, one for moving a student from one homeroom to another (two arrows pointed in opposite directions), and one for removing the student altogether (an arrow coming out of a box).
Click the icon to remove the student, then confirm the dialog box that appears.
With the student removed, if the family has no other children in the school, they will also be removed (i.e. the parents will no longer be listed in the system and they will no longer have access to login.