Weird grading system

Javascript (ES6), 51 bytes

n=>"ABCDEF"[(n<14)+(n<171)+(n<181)+(n<295)+(n<301)]

Alternative solutions (longer):

53 52 bytes (-1 byte thanks to @Arnauld)

n=>"FEDCBA"[n>300?5:n>294?4:n>180?3:n>170?2:+(n>13)]

55 53 bytes (-2 bytes thanks to @Neil)

n=>"AFEDCB"[[14,171,181,295,301].findIndex(m=>n<m)+1]

55 bytes

n=>"FEDCBA"[[13,170,180,294,300].filter(m=>n>m).length]

Example code snippet:

f=
n=>"ABCDEF"[(n<14)+(n<171)+(n<181)+(n<295)+(n<301)]
console.log(f(12))
console.log(f(15))
console.log(f(301))
console.log(f(181))

TI-Basic, 40 bytes

sub("FEDCBA",sum(Ans≥{0,14,171,181,295,301}),1

J, 31 bytes

'FEDCBA'{~13 170 180 294 300&I.

Try it online!

Explanation

'FEDCBA'{~13 170 180 294 300&I.  Input: n
          13 170 180 294 300     Constant array [13, 170, 180, 294, 300]
                            &I.  Use it with interval index to find which of
                                 the intervals (-∞, 13], (13, 170], (170, 180],
                                 (180, 294], (294, 300], (300, ∞) n can be inserted at
        {~                       Index into
'FEDCBA'                         This string and return that char