The IDF has completed their inquiry into the the bombing of the UNRWA school which killed up to 17 people. Their investigation reports that in the during a firefight between the IDF and Hamas terrorists who were located next to the school,  one errant mortar struck the courtyard of the school however by that time the school was completely empty (see video below).

Therefore Israel was not behind the deadly bombing of the school. Although the IDF wont say, I will–that means it was Hamas who killed those 17 Civilians.

July 27, 2014
Location : Tel Aviv
Findings of IDF Inquiry of the UNRWA School Incident in Beit Hanoun
Since Thursday, July 24, 2014, the IDF has conducted a comprehensive inquiry regarding the incident in which the UNRWA school was fired upon. The inquiry concluded that during the intense fighting between IDF forces and Hamas militants, the militants operated adjacent to the UNRWA school. The militants fired anti-tank missiles at IDFsoldiers, who then responded by firing several mortars in their direction.
The inquiry and the documented footage presented here concluded that a single errant mortar landed in the courtyard of the UNRWA school, when it was completely empty. To download the video, right-click here and “save as” [embedded below]

The IDF stresses it does not operate or target international organizations in the Gaza Strip, and the ongoing coordination conducted via the Coordinator of Government Activities in the Territories (COGAT) is continuous without change, even during times of combat.  
In light of the inquiry’s findings, the IDF rejects the claims that were made by various officials immediately following the incident, that people were killed in the school premises as a result of IDF operational activity.