class Student:
    def __init__(self, score=10):
        self.score = score

    def __str__(self):
        return str(self.score)

    def add_score(self):
        self.score  += 10

    def decrease_score(self):
        self.score -= 10

p = Student()

print(p)
10

p.add_score()

print(p)
20

p.decrease_score()

print(p)
10