Once you set the “Attendance” for a calendar event, My School Books will lock that event so that you don’t accidentally move or delete the event. The attendance in My School Books is closely linked with your invoices, so we did this to prevent you from accidentally changing calendar events that have been billed already.

If you need to make a change to a locked event, just edit the attendance and mark all attendees “Unrecorded“. That will unlock the event and you can make the changes you need to.