"Collateral damage is the damage and destruction of targets or personnel not considered as lawful military targets. For instance, accidental bombing of civilian population or medical facilities."
From
Collateral Damage - Glossary Definition - Army Technology
clearly there is no justification for the bombing of the
U.N school. if you are aware of civilians being in the school and bomb and massacre all of them, you DON'T CALL IT COLLATERAL DAMAGE,
("ACCIDENTAL bombing of civilian population"). In no way, shape or form was the school a lawful military target, it was obvious that there were civilians there (its a U.N facility for gods sake!) and as of yet there is no evidence of hamas militants using the school as a military compound.
this is similar to what happened when Israel bombed a U.N compound in Qana in Lebanon, this is better known as the Qana massacre (where additionally, 4 U.N officers were injured and atleast 100 civilians died and another 100 injured)
as you can see this is not an isolated incident and Israel will continue its parade of terror.
and so, we can conclude that Israel has one fucked up, sick 'defence' force