Big difference between public and private schools. Getting expelled from a public school is pretty hard, you basically have to do something seriously illegal involving guns and/or drugs. Getting expelled from a private school is an entirely different thing. They can expel you for a lot of things, and basically anything that makes the school look bad in the eyes of prospective future 'customers' is high on that list. Especially among more prestigious schools.

I don't know, my anecdotes would be from a private school in Europe where a family member is a teacher. They take plagiarism seriously, but also can expel students a lot easier than public schools can.