本人小白，求大佬解答程序为什么会报错

print("欢迎来到学生管理系统！")

students=[]
def add_student():
    print("请输入学生信息")
    num=input("请输入学生学号：")
    name=input("请输入学生姓名：")
    age=input("请输入学生年龄;")
    grade=input("请输入学生的班级：")
    student={"num":num,"name":name,"age":age,"grade":grade}
    students.append(student)
    print("添加成功！")
def list_student():
    print("以下为所有学生信息")
    for i in students:
        num=i['num']
        name=i['name']
        age=i['age']
        grade=i['grade']
        print("学号：%s,姓名：%s,年龄：%s,班级;%s"%(num,name,age,grade))
def find_student():
    num=input("请输入学号：")
    for i in students:
        if i[num]==num:
            num=i['num']
            name=i['name']
            age=i['age']
            grade=i['grade']
            print("学号：%s,姓名：%s,年龄：%s,班级;%s"%(num,name,age,grade))
        else:
            print("输入错误请重新输入")
            continue
def delete_student():
    num=input("请输入你要删除学生的学号：")
    for i in students:
        if i[num]==num:
            num=i['num']
            name=i['name']
            age=i['age']
            grade=i['grade']
            print("学号：%s,姓名：%s,年龄：%s,班级;%s"%(num,name,age,grade))
            a=input("是否要删除")
            if a=="是":
                students.pop('num','name','age','grade')
            else:
                break
        else:
            print("输入错误，请重新输入")
            continue
def main():
    while True:
        print("="*20)
        print("1.新增学生信息")
        print("2.列出所有学生信息")
        print("3.查找学生信息")
        print("4.删除学生信息")
        print("="*20)
        number=input("请输入你要操作的序号：")
        if not number.isdigit() or number not in ['1','2','3','4']:
            print("序号输入不正确！")
            continue
        else:
            if number=='1':
                add_student()
            elif number=='2':
                list_student()
            elif number=='3':
                find_student()
            else:
                delete_student()

if __name__=='__main__':
    main()