The Gaza war, which lasted for 50 days, between Israel and Hamas militants ended on August 26. More than 2,100 Palestinians and 73 people on the Israeli side, 67 of them soldiers were killed in the war.
The human rights watchdog said in a statement that in three cases it examined, Israel caused "numerous civilian casualties in violation of the laws of war", said an AFP report.
The separate shellings of two UN schools in northern Gaza on July 24 and 30, and a guided missile strike on another UN school in the southern city of Rafah on August 3 are the two incidents that prompted the watchdog to make the statement.
"Two of the three attacks Human Rights Watch investigated... did not appear to target a military objective or were otherwise unlawfully indiscriminate. The third attack in Rafah was unlawfully disproportionate if not otherwise indiscriminate", said the watchdog.
"Unlawful attacks carried out wilfully -- that is, deliberately or recklessly -- are war crimes," it said.