Style issues in use of the IF-THEN statement
Note the way the words in these mutually exclusive IF statements line up.

     IF       0 LE Score LT 65 THEN Grade='F';
     ELSE IF 65 LE SCORE LT 70 THEN Grade='D';
     ELSE IF 70 LE SCORE LT 80 THEN Grade='C';
     ELSE IF 80 LE SCORE LT 90 THEN Grade='B';
     ELSE IF       SCORE GE 90 THEN Grade='A';

Other less readable options include:

     IF 0 LE Score LT 65 THEN Grade='F';
     ELSE IF 65 LE SCORE LT 70 THEN Grade='D';
          ELSE IF 70 LE SCORE LT 80 THEN Grade='C';
               ELSE IF 80 LE SCORE LT 90 THEN Grade='B';
                    ELSE IF SCORE GE 90 THEN Grade='A';

But the following is fine:

     IF  0 LE Score LT 65 THEN Grade='F'; ELSE 
     IF 65 LE SCORE LT 70 THEN Grade='D'; ELSE 
     IF 70 LE SCORE LT 80 THEN Grade='C'; ELSE 
     IF 80 LE SCORE LT 90 THEN Grade='B'; ELSE 
     IF       SCORE GE 90 THEN Grade='A';

Readable code is easier to debug and check visually.