This article from Marian Wang at ProPublica is a must read. I have to say, though, I think this doesn't happen only in the juvenile justice system, and very often (wherever it happens) children of color are over-represented.
Though the use of antipsychotic drugs on children is believed to carry significant risks  even when used properly to treat bipolar disorder and schizophrenia, it’s not uncommon in some states  for juveniles in detention to be prescribed antipsychotics simply to counter mood disorders or aggressive behavior, according to an investigation by Youth Today, which covers the juvenile justice system and youth services.
See an article at Youth Today here. Also see my last post for the link to the New York Times article referenced by Wang.