"Did thugs suddenly appear, and the old school organization became incapable of handling it? Or did the way we run schools and society change, and the new "improved" thugs are the result? "

There's always been thugs, and you're right, you used to be able to expel 'em easily.
Society also (maybe) got worse - so now we also see guns in schools.
Expulsion got harder - and I'm just guessing here, I'm sure somebody's done a study - because
a) what do you do with the expellee? Throw him to the streets? Think "No child Left Behind" of yesteryear.
b) Society's parents got more and more litigious.
A PA schoolgirl just successfully sued to be named the sole class valedictorian. Imagine the steps a determined parent would go to with an expelled child.