That's just bullshit.
The US leads the world by a huge margin in mass shootings yet our mental health is the same as theirs.
It's the number and type of guns that are the difference.
^^^ This.
Restrict access to firearms, most of all.
But there's also a secondary issue that I can't get my head around:
Leaving aside the modeling or cluster homicides theory:
Why don't similarly disturbed and disgruntled boys from other countries or from previous American generations express their homicidal impulses in such public ways, and with such a high risk of accountability?
Firearms aren't available for them, but other means are.