It's because schools get sued if they hear something without acting on it.

So a kid mentions that his mom is involved with illegal drugs. Let's imagine the school let's this slide. A few years later, maybe something bad happens with that family. Maybe his mom is arrested for dealing narcotics. Maybe it turns out that the parent is abusing the kid. Or maybe the kid goes on a spree shooting. Who knows? Shitty things happen every day. It doesn't even have to be related to his mom smoking pot.

The community grieves. Reporters descend like fucking vultures on the drama. Another tragedy to fuel the internet's clickbait habit. Eventually it comes out that the kid had a "bad home environment" and the school had evidence of this. People demand to know why the school covered it up. And that's how it'll be framed. Why did they "cover it up" rather than why did they "mind their own fucking business and act like regular humans." An inevitable lawsuit follows, the school district settles, and now the school has a scandalous section on their wiki page.

Or the school can avoid that bullshit and shrug this off to the police and CPS. Liability avoided. It's someone else's problem.

"Let's just avoid a lawsuit" is the basis for a lot of institutional decisions these days. It's why big companies want diversity. It's why employees are fired with at least two other people in the room. It's why teachers report every little thing to their bosses. Every bullying facebook message. Every half-eaten poptart in the shape of a gun. Every violent rap lyric. "Let's just avoid a lawsuit."

TL;DR - Schools aren't wagging their authority dick. They're covering their own asses for liability's sake. Is that cowardly? Sure. But institutions that aren't cowardly get drowned in lawsuits. Eventually only cowardly institutions remain. It's natural selection.