school.clj

(def school (reduce (fn [acc g] (assoc acc g { :grade g :students [] })) {} (range 1 8)))

(defn add-student [name grade school]
  (assoc-in
    school [grade :students]
    (vec (sort (conj (:students (get school grade)) name)))))

(defn add-students [school [name grade]]
  (add-student name grade school))

(defn students-by-grade [school]
  (reduce
    (fn [acc [k v]]
      (if (= (count (:students v)) 0) acc
      (conj acc v))) [] school))


(add-student "Jack" 1 (add-student "Aimee" 2 (add-student "Jack" 2 school)))

; Same as ^

(reduce add-students school {"Jack" 1 "James" 1 "Amy" 5})