Feedback appreciated for my teachers code challenge. Passed but probably overly complicated.

I think most of your code is quite good, your most_courses code is pretty much identical to how I would have coded it. There are a myriad of ways of solving most issues when it comes to programming, some which will be shorter than others, but making the solution as short as possible is not always better.

Shorter code is not guaranteed to be faster than a longer variant, and it can often be harder to read and understand it in the future. So you should not feel bad just because your solution is slightly longer than some of the other answers on this forum. As long as the code is readable and doesn't do a lot of unnecessary stuff you are fine.

That said if I had to come up with some complaints about your code it would mostly be the nested loop that you use in the num_courses and courses function. Looping is a relatively expensive operation that doesn't scale well (the more items the longer it takes) and this becomes even more true when you place a loop within another loop.

It can also easily become hard to keep track of what is happening when you nest loops under each other. Because of that you should generally try to stay away from nested loops whenever you can. Here is an example of the two functions that are written without a nested loop:

def num_courses(teach_dict):
    course_num = 0
    for courses in teach_dict.values(): # Only loop through the values
        course_num += len(courses) # Use the `len` function to get the number of courses
    return course_num

def courses(teach_dict):
    class_list = []
    for courses in teach_dict.values(): # Only loop through the values
        class_list += courses # Append the courses array directly to the class_list array
    return class_list

That is how I would have written those functions. Just as a note though I'm not saying you should never nest loops, there are certainly instances where doing so is the only logical solution, but you should always look at the code and make sure that a nested loop really is the most efficient solution you can come up with.