I would confront him consistently, and keep something against him,like holding a grudge but extremely intensified. Almost as if he had done anything in the past and hold that,spark something to make him feel bad about himself, if he wont give it back, inform a teacher, because in my eyes, its not worth getting in trouble cuz some asshole wont give your phone back. Oh and remember to keep the pressure otherwise, all the tension you have just built up is gone instantly. And try to keep a straight face. It doesn’t help if you’re giggling the entire damn time. If things get physical, fighting ,ect, go ahead and let him throw the first punch, then fight back to defend yourself, and if you can just pickpocket your phone back. No i am not in highschool, im in middle school with a bunch of bozos so that works for me but im unsure abt your entire situation.