A study testing whether schizophrenia in teenagers could be prevented has failed. It didn't tell us the treatments don't work, it did tell us the approach is not practical today.
We need better ways to identify young people with incipient schizophrenia, and safer drugs with fewer side-effects, before we can try this again.
I don't know what makes pyschiatrists think early treatment would prevent development of symptomatic schizophrenia. Maybe it's entirely too early for this sort of study. Do we really know enough about the disorder?