Maybe the criterion should have been native speaker of German because this could be a bit harder to figure out if you aren't. The closest among the answers is "downhill," although I don't think that you'd hear an 11-year-old kid saying this.
It's about "sozialer Absturz" (you've given enough context, IMO):
http://www.spiegel.de/lebenundlernen/schule/schulstudie-bild...https://www.welt.de/politik/deutschland/article160768967/20-...https://bfriends.brigitte.de/foren/depressionen/454058-sozia...http://forum.f-sb.de/showthread.php?50804-totaler-sozialer-a...Nothing to do with getting angry in any shape or form. It's about a plunge, downfall or plummet. If it weren't a kid talkin', I'd say "take a nosedive" (figuratively speaking) could be pretty much it, although it's more about social status and less about some kind of emotional state (i.e., neither wreck, nor lose it, nor flip out will do).
Please be aware that "...ist abgestürzt" describes an
external phenomenon (i.e. what you can observe; no state of mind).
Best
peer agreement (net): +4