The Custom Attendance Report now allows the functionality to show whole school attendance including the X code (absent due to exceptional circumstances). If you go to Reporting > Reports > Attendance (tab) > Custom Attendance Report
Set the dates you require and there is an option to select 'Include x Codes as Authorised Absences' (above the Choose button).
Once the report is run, all X codes will be counted as absent to more accurately reflect the overall attendance percentage. There is a disclaimer to say these figures do not conform to the DfE calculation for attendance, so for official purposes the report will need to be run excluding the X code.