Both the Parkland and the Santa Fe shooters apparently sported Fascist and/or Communist signs. The Santa Fe kid also apparently wore some satanic images. He wore a trenchcoat every day. He apparently spent a lot of time alone in an abandoned mobile home.
Most of these shooters–perhaps all of them–showed signs indicating that acting out violently was at least possible for them. The signs need not relate directly to threats. It was found many years ago, for example, that most serial killers start as small time arsonists and torturers of small animals like cats. If you’re killing cats, you have problems.
Likewise, some sort of rough algorithm can and should be devised for both schools and local police to narrow down who is potentially trouble. Now, this list might be scary long. This is a point I have made. Just about every school in the country likely has several kids who might fit the bill. We are a fucked up nation. That is the long and short of it.
But early action is likely our best solution to this continuing problem. None of these kids snap overnight. It is always like water boiling. It takes time, and it is not invisible in nearly all cases.