for.py

#!/usr/bin/python3.6

school_scores = [{'school_class': '4a', 'scores': [3,4,4,5,2]},
    {'school_class': '5a', 'scores': [3,4,4,5,2,4]},
    {'school_class': '10a', 'scores': [3,4,4,5,2,5]},
    {'school_class': '10b', 'scores': [3,4,4,5,2,2]},
    {'school_class': '11e', 'scores': []},
    {'school_class': '1a', 'scores': [3,4,4,5,2,1]}]

global_scores=[]

def get_average_from_list(scores_list):
    list_members_sum=sum(scores_list)
    list_members_amount=len(scores_list)
    try:
        average_rate = list_members_sum / list_members_amount
    except ZeroDivisionError:
        average_rate = None
    return(average_rate)

for class_dict in school_scores:
    class_name = class_dict.get("school_class")
    class_scores = class_dict.get("scores")
    class_average_rate = get_average_from_list(class_scores)
    print (class_name, "average: ", class_average_rate)
    global_scores.extend(class_scores)

# since i dont trust to average from few averages, 
#     i use global_scores list to get average for whole school.
school_average = get_average_from_list(global_scores)
print("School average: ", school_average)