$$ C(n, r) = \frac{n!}{(n-r)! \cdot r!} $$
where:
n is the total number of items
r is the number of items to be selected
In this case, n = 9 (the number of subjects) and r = 5 (the number of classes in a schedule). So, the number of possible schedules is:
$$ C(9, 5) = \frac{9!}{(9-5)! \cdot 5!} = \frac{9!}{4! \cdot 5!} = 126 $$
Therefore, there are 126 possible schedules of 5 classes.