Hmm. I don't know what you mean by his comment about "time" (I don't see time as a factor in your response?), but I would interpret your response as more of a "can lead a horse to water but can't make him drink" or "I wash my hands of kids who don't put the effort in" type of response. While it's true that you can't force kids to pass, administrators might be interested in hearing more about what you would do (or have done) specifically to help students who are resistant to doing work. Your answer, if you really said what you wrote, can be interpreted as you being someone who isn't going to put much effort into helping struggling students because you feel it's just natural that some will fail. I think focusing on what you'd do to prevent it (without saying that it's inevitable that some will fail) would be a more positive response. We all know some will fail. It happens. But when you say that straight in the interview, your response is probably a lot less positive than other applicants.
*I didn't read as closely your last paragraph before I wrote this post- I see you mention the horse/water analogy, too
