Fixing the format
What is supposed to be done with the values that fall between the cracks? In our example, 89.1? Go back to the definition of the task:
"The task is to write a recoding algorithm that assigns below 65 to F, below 70 to D, below 80 to C, below 90 to B, and 90+ to A."
That is, "below 65" implies that 65.0 is a D and 64.9999 is an F.
The original PROC FORMAT is as follows.

PROC FORMAT;
VALUE SCORE  0-64  ='F'
            65-69  ='D'
            70-79  ='C'
            80-89  ='B'
            90-HIGH='A';

The revised VALUE statement is as follows:

VALUE SCORE  0-<65  ='F'
            65-<70  ='D'
            70-<80  ='C'
            80-<90  ='B'
            90-HIGH ='A';

The new Proc Print works as you'd want it to.

Example 3, with different data

OBS    ID    SCORE     GRADE    NOFORMAT

 1      1    F           F          0.0
 2      2        -1                -1.0
 3      3    A           A        101.0
 4      4    B           B         82.1
 5      5    B           B         89.1
 6      6         .                  .