Neither of those serbian shootings were in schools. NYC has had shootings though not that many: https://en.wikipedia.org/wiki/List_of_shootings_in_New_York_...

Also isnt there recent history of people going to different states to buy guns to shoot people in another state? Its not like there are border police checks

The argument was that Serbia - a tiny, war-traumatized country -tells you something about current or potential US gun regulation. My response is that even if you allow for that degree of cherrypicking, the comparison doesn't you get you anything that supports the GP's position, in fact, it gets you the opposite.