VBA will hide columns, but not unhide them

Try this:

Private Sub Worksheet_SelectionChange(ByVal Target As Range)
    
    With Worksheets("Attendance & Organisation")
    
      'Unhides all columns
      .Range("AA:ZZ").EntireColumn.Hidden = False
    
      'Hides specified columns as provided by ZZ
      If .Range("ZZ1").Value = 100 Then
          .Range("CO:OF").EntireColumn.Hidden = True
      ElseIf .Range("ZZ1").Value = 101 Then
          .Range("O:OF").EntireColumn.Hidden = True
      ElseIf .Range("ZZ1").Value = 102 Then
          .Range("K:N,Y:OF").EntireColumn.Hidden = True
      End If
    
    End With
    
End Sub