The weapon fell on Terra Santa Middle School on Saturday evening whilst a number of people, including many elderly, were inside.
It's thought people had come to the school to seek refuge from the bombing of forces loyal to President Al-Assad.
The school had been considered a safe place that would not be targeted because of its religious links.
Classes were run by Franciscan monks who had opened up some classrooms as a make shift refuge centre.
The missile shot through the wall as people gathered in a hall.
School principal Br. Firas Lutfi said: "The explosion was extremely violent, and it was strongly felt.
"Apparently there is a no longer one place in Aleppo that is 100 percent safe."