Study to Evaluate Weight Gain as Assessed by Change in BMI Z-score in Pediatric Subjects With Schizophrenia or Bipolar I Disorder

Inclusion Criteria: * Subjects aged 13 to 17 years with schizophrenia or aged 10 to 17 years with bipolar I disorder, diagnosed according to Diagnostic and Statistical Manual of Mental Disorders, 5th Edition (DSM-5) criteria * Subject is an outpatient or will be able to be treated on an outpatient basis (per Investigator judgement) by study Week 2 * Subject has reliable family/legal guardian support available for outpatient management * Subject is either currently treated with olanzapine, or if treated with another antipsychotic, the subject has had an inadequate response (eg, unsatisfactory clinical response, AEs, or nonadherence to current medication) based on Investigator judgment * Subject must not be a danger to self or others (per Investigator judgement) Exclusion Criteria: * Subject presents with a major depressive episode(bipolar I disorder) or other neuropsychiatric diagnosis (according to DSM-5 criteria) including schizoaffective disorder, current major depressive disorder that is untreated and/or unstable, or any other psychiatric condition that could interfere with participation in the study * Subject has a history of seizure disorder (exception: history of febrile seizures), severe head trauma with loss of consciousness within the 12 months prior to Screening, or other clinically significant neurological condition within the 12 months prior to Screening * Subject poses a current suicide risk as assessed by the Investigator or as confirmed by the baseline Columbia-Suicide Severity Rating Scale (C-SSRS) * Subject has received olanzapine for \>= 14 days during the month prior to screening, or has a history of poor or inadequate response to treatment with olanzapine * Subject has taken opioid agonists within 14 days prior to Screening, or within 30 days prior to Screening (for long-acting opioid agonists) * Subject anticipates needing to take opioid medication during the study period (eg, planned surgery, including oral surgery) * Subject has taken opioid antagonists including naltrexone (any formulation) or naloxone within 60 days prior to Screening * Subject has used a long-acting injectable antipsychotic medication within 3 injection cycles prior to Screening * Subject has a BMI percentile \>98th or \<5th * Subject has a diagnosis of diabetes mellitus or presents with prediabetes lab results at Screening (hemoglobin A1c \[HbA1c\] \>= 6%) * Subject has started a smoking cessation program within the 6 months prior to Screening or has joined a weight management program or has had significant changes in diet or exercise regimen within 6 weeks prior to Screening * Subject has participated in a clinical study of an investigational product within the last 30 days prior to Screening