import bisect
from collections import defaultdict
from typing import List, DefaultDict


class School:
    def __init__(self) -> None:
        self.student_db: DefaultDict[int, List[str]] = defaultdict(list)

    def add_student(self, name: str, grade: int) -> None:
        bisect.insort(self.student_db[grade], name)

    def grade(self, grade_number: int) -> List[str]:

        if grade_number not in self.student_db.keys():
            return []

        return self.student_db[grade_number]

    def roster(self) -> List[str]:
        return sum([self.grade(grade_number) for grade_number
                    in sorted(self.student_db)], [])