So NOW he IS a student. Yes, he was NOT a student before he WAS a student. He was CHINESE before he was KOREAN. So it naturally follows that the poor boy was TROUBLED. But remember, this was NOT a terrorist act!
The explanation is that they found a cell phone near Cho's body that belonged to Chinese exchange student, and went down the wrong path for a while.
If he was a student and he was Korean, he was both yesterday and the day before. Probably longer than that. The story changes as facts become available and discernible from rumor -- the facts don't change.
Always apply a grain of salt to any news reports out of a chaotic situation for at least the first 24 hours. In the Centennial Olympic Park bombing, the wires were quoting people saying that dozens of people were killed. Final death count -- one or two, depending on whether you count the Turkish cameraman who had a heart attack running to the scene with his gear. That doesn't mean that dozens of dead rose again. It means that panicked witnesses were mistaken, and the people lying on the ground not moving much were merely wounded.