Paste this formula into the column to the right of your numeric scores, and watch it go to work.
Thanks to Brad Denney, the formula now works down to F. Because it calculates plus and minus
after determining the letter, it gets around the problem of the limit on strings of IF
statements I had before. Using this formula as is requires that Excel be set to the R1C1
reference style (see Tools_Options_General).

=IF(C[-1]<60,"F",IF(C[-1]>=90,"A",IF(C[-1]>=80,"B",IF(C[-1]>=70,"C","D")))&IF(MOD(C[-1],10)>=7,"+",IF(MOD(C[-1],10)>=3,"","-")))

Note that this formula can easily be modified by changing the cutoffs or outcomes. However,
because of the plus and minus issue, this formula will treat 60-69 the same as 70-79, etc.