Group Sorting takes precedence over Record Sorting.
Here's what you need to do:
1) Go to the Insert Summary option > Choose the formula you created to convert to number as the 'field to summarize' > Choose 'Maximum' as the sum function > Under 'Summary Location', choose Group Footer (Grade group)
2) Go to the Group Sort Expert > Select the Grade Group > Under 'for this group sort', choose 'All'
3) Under 'Based on', select the summary field and set the sort order as 'Descending'.
Hope this helps.
-Abhilash