I think it depends on the situation really. Self harm is one of those things that requires you have extensive knowledge about what self harm even is, extensive knowledge of the person doing it and their reasons behind it. Parents and teachers aren't generally going to have much input on helping someone stop doing so. The most they could do is refer someone to a doctor. Razors and knives are so easily accessible anyways so it's not as if they had the power to stop someone from obtaining one.
So really I would doubt someone would "completely ignore" someone else's self harming but they wouldn't have an answer because it's a very complicated issue to deal with and most people honestly just don't know how.
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If Jesus died for my sins, then I should sin as much as possible, so he didn't die for nothing.