I might have coded it this way.

grader.hs

import Data.Char (isAlpha, toUpper)
import System.Environment (getArgs)

-- These are the correct answers we expect to see.
correctAnswers = "BDAACABACDBCDADCCBDA"

main = do 
  args <- getArgs -- We require 1 argument
  cont <- readFile (head args)
  let answers  = filter isAlpha cont
      theScore = score answers
    in do putStr (grade theScore)
          putStrLn $ " (Score was " ++ (show theScore) ++ ")"
   
score ::  String -> Int
score = length . filter correct . zip correctAnswers . map toUpper
        where correct p = fst p == snd p

grade :: Int -> String
grade score 
  | score >= 15 = "Passed"
  | otherwise   = "Failed"
  
  

simpleInput.txt

BDAACABACDBCDADCCBDA

trickyInput.txt

1. B
2. D
3. A
4. A
5. C
6. A
7. A
8. A
9. C
10. D
11. B
12. C
13. D
14. A
15. D
16. C
17. C
18. B
19. D
20. A

To build:
ghc --make grader

To run:
./grader trickyInput.txt