He gets in a motorcycle accident and comes close to death in one episode and then he's never seen or anything again until the episode where they graduate from high school over a year later. Does he make it through? Does he move away? Does he die? (I thought he died). Does he have to stop teaching because of rehab or something from the accident? What?
So, what you're saying here is that he was seen in this episode? If that's the case, then it's quite obvious he didn't die (actually, he isn't seen - only mentioned as being in the area, but not on screen). The truth is, this is one of those famous television plots that were never resolved. There was never an end to Mr. Turner's story, but all we know is that he did survive.
A kid from the first season comes back on the show for the last few episodes when they're in high school. When they ask where he's been he points off camera and says over there...in a part of the school that's never seen and supposedly no one ever comes back from...or so says Shawn and Cory. The kid laughs and says that's silly and then says "Hey, Mr. Turner, wait up" and walks off screen. You never see Mr. Turner after the accident episode.
They most likely made something happen to him, because his contract was up and they weren't going to renew it or something. He may come back on, but if not, you'll never know what happened.
That's what I said ... in less words.