According to Israel, they were going after militants on a motorbike when the school was hit.
"The Israel Defense Forces said that it targeted three Palestinian Islamic Jihad members riding a motorcycle in the vicinity of the school. The IDF said it is reviewing the consequences of the strike but didn't say if the suspected militants were hit."
http://www.cnn.com/2014/08/03/world/...html?hpt=hp_t1