Update: This was fixed in the 1/30/2020 update:
Known Issue - Assertive Discipline page will generate an error if the school does not have a calendar (DAY table) such as an Inactive school. Also an error will generate when logged into the district and going forward or backward and the next student is in the Inactive school.
A workaround now is to copy a Calendar (DAY table) from a valid school.
The fix is scheduled to ship in the next update planned for this week.