text: class School:
      def __init__(self):
          self._db = {}
  
      def add_student(self, name, grade):
          self._db[grade] = self._db.get(grade, [])
          self._db[grade].append(name)
          self._db[grade].sort()
  
      def roster(self):
          tmp = []
          for grade in sorted(self._db.keys()):
              tmp = [*tmp, *self._db[grade]]
          return tmp
  
  
      def grade(self, grade_number):
          return self._db.get(grade_number, [])