Grade school exercise tests too implementation specific about common-lisp HOT 5 CLOSED

(in-package #:cl-user)
(defpackage #:school
  (:use #:common-lisp)
  (:export #:school #:add #:grade-roster #:grade #:sorted))
(in-package #:school)

(defparameter *students*
  ()
  "Student data")

(defun add (student-name student-grade)
  (push (cons student-name student-grade) *students*))

(defun grade (grade)
  (mapcar #'car (remove-if-not #'= *students* :key #'cdr)))

(let ((*students* ()))
  (add "Jim" 2)
  (assert (equal *students* '(("Jim" . 2))))
  (assert (equal (grade 2) '("Jim"))))

(defun grade-roster ()
  (sort (copy-list *students*)
        (lambda (student1 student2)
          (or (< (cdr student1) (cdr student2))
              (string< (car student1) (car student2))))))

(let ((*students* '(("Alex" . 2)
                    ("Zoe" . 2)
                    ("Anna" . 1)
                    ("Peter" . 2)
                    ("Barb" . 1)
                    ("Charlie" . 1))))
  (assert (equal (grade-roster)
                 '(("Anna" . 1)
                   ("Barb" . 1)
                   ("Charlie" . 1)
                   ("Alex" . 2)
                   ("Peter" . 2)
                   ("Zoe" . 2)))))

;;; Bonus points, basic.

(let ((*students* '(("Josie" . 1)
                    ("Joseph" . 1)
                    ("Jose" . 1)
                    ("Joe" . 1))))
  (in-package #:cl-user)
  (multiple-value-bind (#:out err)
      (ignore-errors
        (push '("Chris" . 2)
              (find-symbol "*students*"
                           (find-package 'school))))
    (check-type err error)))