When you view a classroom page, the students in the classroom are listed on the left side of the page. Next to each name may be a small envelope icon. If a child does not have an envelope icon, then that student does not have any parents with email addresses in the system.
This may be because the student parent information was never added to ClassroomParent, or it may be that email addresses were not supplied with the parent information associated with the student.
Any student with an email icon beside their name has at least one parent with an email address.