The purpose of this study is to determine the safety, efficacy, and pharmacokinetics (PK) of etrasimod for the treatment of moderately to severely active ulcerative colitis in adolescents (≥ 12 years up to \< 18 years of age). Participants who will complete the total 52-week treatment period will have the opportunity to continue in a Long-Term Extension (LTE) Period of up to 4 years (5 years after study enrollment).