Why is this not correct?

# The dictionary will look something like:
# {'Andrew Chalkley': ['jQuery Basics', 'Node.js Basics'],
#  'Kenneth Love': ['Python Basics', 'Python Collections']}
#
# Each key will be a Teacher and the value will be a list of courses.
#
# Your code goes below here.

def num_teachers(teachers):
    return len(teachers)

def num_courses(teachers):
    courses = 0
    for teacher in teachers:
        courses += len(teachers[teacher])

    return courses

def courses(teachers):
    courses = []

    for teacher in teachers:
        for course in teachers[teacher]:
            courses.append(course)

    return courses


def most_courses(teachers):
        courses = 0
        busiestTeacher = ""
        for teacher in teachers:
            if courses < len(teachers[teacher]):
                courses = len(teachers[teacher])
                busiestTeacher = teacher

        return busiestTeacher