FERPA defines "education record" as records containing information directly related to a student and which are maintained by an educational agency or institution or by a party acting for the agency or institution. One of the exceptions to that definition is a "sole possession record." In order to meet the criteria for exclusion as a sole possession record, the record must be kept only in the sole possession of the maker, be used as a memory jogger, and not be accessible or revealed to another party except a temporary substitute.
A grade book does not meet the stated criteria. The information in the grade book is usually shared with other school officials; it isn't used only as an aide to memory; and it's revealed to many parties, at least in portions, to students and parents. The Family Policy Compliance Office in Washington, D. C., which administers FERPA, defines a grade book as multiple education records, in fact. The information and grades of each student are considered the education record of each of the individual students. In order for an instructor to properly disclose the grades for an individual student, the instructor would need to redact or otherwise cover the other student's grades when viewed by the student.