It would be different for two reasons:
1. PTSD patients aren't as stigmatized, dismissed, dreaded, and avoided by our medicine colleagues.
2. Once it's in the chart, it's in the chart and in the notes of colleagues, so erasing it makes no difference as problems lists are routinely so long that people don't double-check every single psych diagnosis. Once someone in the ED writes a note that includes BPD, the next time the patient comes in, they carry that forward without even realizing that the patient may no longer meet criteria.
As can I. See above.
Acute circumstances can last much more than a year or two for a 13 or 14 or 15-year-old. We're talking peer bullying, childhood abuse at home, drama in the peer group, etc, magnified by normal adolescent stress intolerance and emotional dysregulation.
I'm not convinced it's the "vast majority" of their peers, honestly. Have you ever taught middle school students (primarily girls)? There are quite a lot who have anything but a peaceful co-existence with their peers. Add to that one or two who have had turbulent upbringings and they're already pre-disposed to maladaptive coping skills. Add in hormones and impulsivity that comes with that age group and you've got the makings of a personality that could meet criteria for BPD without the understanding that the situation is transient and the child's personality is likely to be as well (and yes, I've read the literature about personality being formed by the age of 6, with which I disagree).