Edit | Delete marks column
This option allows you to remove one column from the grid.
The grid is put into editing mode. If part of the grid appears to be selected, ignore it - it depends on previous operations. The status bar gives the instruction - place the cursor on the column you wish to delete, then click that column. A warning message appears; if you select No, the system is reset and your click is ignored. If you select Yes, the column is immediately deleted, and the grid returns to non-editing mode.
Note that you can't remove the fixed column of names, nor can you remove the column of GCSE (or Target) scores. If you wish to do this, you will need to start a new file, e.g. by exporting to Excel, deleting the column there, and re-importing.
Note that this deletion is from the displayed grid, not the file. If you have made a mistake, you can at this point re-open the file (File | Open) but do NOT save the file first! (You will be prompted to save the file - select No in this case). Note also that if you have to re-open the file in this way, the version to be re-opened will be the last version saved. It's a good idea to save the file before deleting a column, and maybe take a backup.