True hallucinations and schizophrenia are rather rare with autism, but the true prevalence may be difficult to know as non-verbal autistics may not report what they perceive.
There is some evidence that bullying may lead to hallucinations, and that persons who feel isolated may also perceive hallucinations. Those with social anxiety may develop unwarranted paranoia. Alcohol and or substance abuse may also lead to hallucinations, usually limited to usage, but occasionally in the after-effects. Autistics often have fine-tuned senses and may have difficulties discerning where a sound or voice is coming from, but this is not a true hallucination. Although rare, autistics can develop schizophrenia, usually as young adults and the diagnosis and treatment are as with any person with schizophrenia.