I'm surprised the number of self harmers is so high. I've known a couple of people, my age, who self harmed at times in their lives when they were in great distress. Cuts and cigarette burns. Can't say I knew any at school which doesn't mean there weren't any.
It's very worrying to think so many are doing it now.
Many of those who completed the survey may only have done it once, doesn't mean they are persistent self harmers.