Setup a table like this

0    Very Low Achievement
0.25    Low Achievement
0.45    Sound Achievent
0.65    High Achievement
0.8    Very High Achievement

I set this up in B6:C10

Then, after calculating a students % score use the vlookup function as follows
=VLOOKUP(C15,$B$6:$C$10,2,TRUE)

The C15 is where I placed the score to lookup.  The absolute references
$B$6:$C$10 are to ensure that you can copy the formula to adjacent cells
properly. The true value is a range lookup, the 2 refers to the second column
of the table from which we display the results.

You might need to play a little with the numbers is column B of the table to
deal with a score that hits the % boundaries exactly