with open("file.tsv", "r") as f, open('out.txt', 'w') as o:
    a = {str(i): [] for i in range(1, 12)}
    [a[i[0]].append(int(i[2])) for i in map(lambda x: x.split('\t'), f.read().split('\n')) if i[0]]
    o.write('\n'.join([' '.join((i, str(sum(a[i])/len(a[i]) if len(a[i]) > 0 else '-' ))) for i in map(str, range(1, 12))]))

Traceback (most recent call last):
  File "test.py", line 16, in <module>
    value = [i + 1, ' ', t[i] / t2[i], '\n']
ZeroDivisionError: integer division or modulo by zero

 with open("myfile.txt") as file:
    students = {str(i): [] for i in range(1, 12)}
    for line in file:
        line = line.strip().split('\t')
        person = line[2]
        key = line[0]
        if key in students:
            students.get(key).append(person)
    for key in students:
        sum = 0
        size = len(students.get(key))
        if len(students.get(key)) > 0:
            for elem in students.get(key):
                sum += int(elem)
            print(key,round(sum / size, 5))
        else:
            print(key, '-')