As pan64 says, put all "read" statements only
into the loop.
Furthermore you have too many tests within the elif statements. If a number is already tested to be greater then 50 within the while loop, you'll only have to check if it is less than 60, because it cannot be less than 50 at this line of the while loop. It should be
if [GRADE -lt 50 ]
elif [ GRADE -lt 60 ]
elif [ GRADE -lt 70 ]