Had to check multiple times to pass the task? Task 4

My complete code.

def most_classes(x):
    high = 0
    current = 0
    name = None
    for item in x:

        if len(x[item]) > current:
            high = len(x[item])
            current = high
            name = item
        else:
            current = len(x[item])
    return name
def num_teachers(y):

    return len(y)

def stats(z):
    ourlist = []
    containerlist = []
    for item in z:
        containerlist = [item, len(z[item])]
        ourlist.append(containerlist)

    return ourlist

def courses(zz):
    courselist = []
    for item in zz.values():
        courselist.extend(item)

    return courselist

I've tested all of this through the workspace running each part multiple times and it always gave me the correct value. But for me to get task 4 to pass in the challenge I had to check the task at least 10 times before it accepted it. Is there something wrong with my code? If there is something wrong could you explain to me why it's wrong so I can better understand and not make the mistake in the future?

Also, and aside from the variable names I know those are horrid, is my code itself okay? I'm new to this whole thing and would like to know if I'm progressing well.

Thanks in advance.