Set the Monday of the first teaching week (Class 1). The weekly calendar view will display Class X based on the number of weeks since this date.
Mark weeks where no classes are held (e.g. school holidays). Select the Monday of the week. These weeks will be greyed out in the calendar and skipped in the Class numbering.
No No-Class weeks configured yet.
Create a backup of the current database. The backup will be saved as backup_DD-MM-YY.sql.
Restore the database from a previous backup. This will overwrite the current database.