Let me just get this clear - you want the max(grade) from the data returned by that select statement? Do you want all the rest of the data returned by the query as well? if not, just replace the "*" (which is bad SQL design anyway) with "max(grade)".
If you do want the rest of the data (I'm guessing you do) and you don't want to make the query again, you'll have to get PHP to crawl through the result sets for it.
Annoyingly, you can't go "SELECT *,max(grade) FROM ...." because that would be mixing rows of data with aggregate functions, and for some reason having
Country | ... | grade | max(grade)
SENEGAL | ... | 2 | 4
FRANCE | ... | 1 | 4
JAPAN | ... | 4 | 4
Is a no-no.