Why do school shootings happen mostly in the US?

It's all about bad gun control legislation, based on a wrong interpretation of the Second Amendment.

While most countries have strict and adaptive gun control legislation, in USA the Second Amendment to the Constitution is interpreted in a wrong way, allowing almost everyone in the US to possess heavy guns.

Weak gun control legislation in the US leads to a situation where any person, without being checked for appropriate mental state, can buy a heavy rifle and go on a human safari. The powerful gun lobby in the Congress prevents this situation from changing.