Using schools to launch attacks against those you don't like.
Imagine if that happened in America!
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Note that it was counter battery mortar fire beyond visual range. This was an immediate response since Hamas mortar teams fire and then move. There is no time to try for positive ID and in any case, the mortar fire MUST be suppressed to save Israeli lives!
That's where they learned it from, as foreign exchange students. They've just taken it a step further from 'political attacks' and escalated it to terrorist rocket attacks.