To view Attendance Notes, users need Read permissions to Attendance Notes under Attendance/Enrollment section in Security.
The minimum permissions required for Attendance Notes to be added are shown in the screenshot below. Giving the user permissions to the Update will allow them to use the Include Attendance Note option on the Mass Change Period Absences form.
Portal Options need to be configured for teachers to view and/or add Attendance Notes. These options are in addition to the Security permissions above.