Рост учеников в классе

a, b = [], [[0 for i in range(1, 4)] for j in range(1, 12)]

with open('file.txt') as file:
    for i in file:
        i = i.strip().split()
        a.append(i)

for i in range(11):
    for j in range(len(a)):
        if int(a[j][0]) == i+1:
            b[i][0] += int(a[j][2])
            b[i][1] += 1

for i in range(11):
    if b[i][1] == 0:
        b[i][2] = '-'
    else:
        b[i][2] = b[i][0]/b[i][1]

for i in range(11):
    print(i+1, b[i][2])