Excel IF statement Not returning the appropriate Value

This is the formula am having...trying to grade student by giving them there A's and B' defending on there score and to return a value of the cell if someone is having "absent" instead of a score...but it is not returning the value of the cell... the referenced record its in a separate sheet called "raw" and it got me thinking or may be its because its a string data am trying to return...still don't know please somebody should help ASAP...am using excel 2007...Thanks for trying

=IF(raw!E6>=75,"AA",IF(raw!E6>=70,"AB",IF(raw!E6>=60,"B",IF(raw!E6>=50,"C",IF(raw!E6>=40,"D",IF(raw!E6<40,"RT",raw!E6))))))