Whats wrong with my code, I just can't get my head around it??

The error comes up with 'get_class_average([alice]) resulted in an error: list indices must be integers, not str '

lloyd = {

"name": "Lloyd",

"homework": [90.0, 97.0, 75.0, 92.0],

"quizzes": [88.0, 40.0, 94.0],

"tests": [75.0, 90.0]

}

alice = {

"name": "Alice",

"homework": [100.0, 92.0, 98.0, 100.0],

"quizzes": [82.0, 83.0, 91.0],

"tests": [89.0, 97.0]

}

tyler = {

"name": "Tyler",

"homework": [0.0, 87.0, 75.0, 22.0],

"quizzes": [0.0, 75.0, 78.0],

"tests": [100.0, 100.0]

}

# Add your function below!

def average(numbers):

total = float(sum(numbers))

result = total / float(len(numbers))

return result

def get_average(student):

homework = average(student['homework'])

quizes = average(student['quizzes'])

tests = average(student['tests'])

grade = (.1 * homework) + (.3 * quizes) + (.6*tests)

return grade

def get_letter_grade(score):

if score >= 90:

return "A"

elif score >= 80:

return "B"

elif score >= 70:

return "C"

elif score >= 60:

return "D"

else:

return "F"

print get_letter_grade(get_average(lloyd))

def get_class_average(students):

results = []

for student in students:

results.append(get_average(students))

print average(results)