Clean up grade-school exercise (#417)

* grade-school: Remove unused function from example.el

* grade-school: Fix stubs arguments

- use school instead of students in line with exercise description and
  `example.el`
- fix `add` function stub missing two arguments
- fix `grade` function stub having wrong order of arguments

* grade-school: Fix flycheck warnings in tests

* grade-school: Provide stub for `make-school` function called by test

If we provide stubs then the students will reasonably expect us to
provide stubs for all the functions expected / called by the test
code.

exercises/practice/grade-school/.meta/example.el

@@ -28,9 +28,6 @@
 (defun grade (school grade)
   (gethash grade (school-roster school)))
 
-(defun set-grade (school grade newval)
-  (puthash grade (sort newval #'string<) (school-roster school)))
-
 
 (provide 'grade-school)
 ;;; grade-school.el ends here

exercises/practice/grade-school/grade-school-test.el

@@ -6,9 +6,10 @@
 
 
 (load-file "grade-school.el")
-(declare-function roster "grade-school.el" (students))
-(declare-function add "grade-school.el" (students))
-(declare-function grade "grade-school.el" (desired-grade students))
+(declare-function make-school "grade-school.el")
+(declare-function roster "grade-school.el" (school))
+(declare-function add "grade-school.el" (school name grade))
+(declare-function grade "grade-school.el" (school grade))
 
 
 (ert-deftest roster-is-empty-when-no-student-is-added ()

exercises/practice/grade-school/grade-school.el

@@ -4,14 +4,16 @@
 
 ;;; Code:
 
+(defun make-school ()
+  (error "Delete this S-Expression and write your own implementation"))
 
-(defun roster (students)
+(defun roster (school)
   (error "Delete this S-Expression and write your own implementation"))
 
-(defun add (students)
+(defun add (school name grade)
   (error "Delete this S-Expression and write your own implementation"))
 
-(defun grade (desired-grade students)
+(defun grade (school grade)
   (error "Delete this S-Expression and write your own implementation"))