Schizophrenia can be hard to diagnose for a few reasons. One is that people with the disorder often don't realize they're ill, so they're unlikely to go to a doctor for help.
Another issue is that many of the changes leading up to schizophrenia can mirror other normal life changes. For example, a teen who's developing the illness might drop a group of friends and take up with new ones. The same teen may also have trouble sleeping or suddenly start coming home with poor grades. These symptoms may occur for weeks, months and even years before full psychosis occurs.