It was said to have been "subjected to bombing and destruction by the Israeli war machine."
Here is what it looks like now:
Here is the damage it sustained in May:
I don't have any photos of the inside damage to see if the "spray pattern" of shrapnel characteristic of Gaza rockets can be seen, but this looks like it came from a fairly small rocket.
Given that Israel doesn't aim airstrikes at schools unless there is clear evidence of a military target there, and when they do target something or someone they will use a much larger explosive than this, I believe that this school was hit by a rocket that came from one of the Gaza terror groups, one of hundreds that fell short in Gaza.