Even Al Jazeera, a clearly biased news source when it comes to this subject, included the caveat that Israel contends it was in fact Palestinian rocket fire that hit the building. Do you think Israel is stupid enough to intentionally fire on a UN facility?
The Gaza health ministry told the Reuters news agency that Israeli fire had killed at least 15, and injured 150, at the school in Beit Hanoun on Thursday. .
An Israeli military source however told Al Jazeera that Palestinian rocket fire had been detected in the area and that it might have fallen short and hit the shelter.
Al Jazeera's Stefanie Dekker, reporting from Gaza, said she was unable to reach the school after the attack due to heavy Israeli shelling. No one she had spoken to in Gaza believed the deaths were caused by a Palestinian rocket.
The reporter was near the field, so an eyewitness has more impact than what the military source said.
To add onto that
"We gave the Israelis the precise GPS coordinates of the Beit Hanoun shelter. We were trying to coordinate a window [for evacuation] and that was never granted," he said.
They were trying to negotiate an evacuation with the Israeli military but were ignored leading one to believe that it could well have been Israel.
But fair enough another article from Time does validate your point
http://time.com/3030301/gaza-school-israel-palestine-united-nations/U.N. High Commissioner for Human Rights Navi Pillay said Wednesday there was a “strong possibility” that Israel was committing war crimes in Gaza while also condemning the indiscriminate rocket fire from Gaza. UN Secretary-General Ban Ki-moon has expressed “outrage and regret” after rockets were found to have been stored inside a UN building in Gaza.
An earlier version of this story incorrectly stated that the attack on the Gaza shelter was committed by Israel. The source of the attack has not yet been confirmed.
http://www.reuters.com/article/2014/07/23/us-palestinians-israel-un-rockets-idUSKBN0FS2FF20140723(Reuters) - U.N. Secretary-General Ban Ki-moon expressed alarm on Wednesday that 20 rockets found hidden in a United Nations school in the Gaza Strip had gone missing and directed the world body to deploy experts to deal with the situation.
The main U.N. agency in Gaza, UNRWA, found the rockets in one of its vacant schools a week ago. It found a second batch in a vacant school on Tuesday, but said in a statement that because staff were withdrawn quickly, they were "unable to confirm the precise number."
If that intel is correct there is a reason the rockets could have targeted that building as well.