Umm... I need help with this situation?

Ok my whole grade went on a trip to the Bronx Zoo. On our way back my friend found out her phone was stolen on the bus ride back to school. She lent it to Sarah, Sue, and Jen. (these are fake names) She remembers Sarah giving back to her, besides Sarah hates thieves and always borrows her phone and ALWAYS returns it. Sue has know my friend since first grade so even if she stole it to freak her out she would have returned it a minute later. Jen, steals stuff, loves to pick up fights, and is pretty bitchy. We also think it's Jen because we tried texting to the phone and the person on the phone said their name was Veronica. in gym Jen said that she always wished her name was Veronica and used it as a fake name to trick the gym teacher. Plus every time someone mentions Jen to "Veronica" she never replies again. So we're pretty sure it was her. But she's being extra defensive and we have no idea how to get the phone back. What do you think? Do you think she did it? If so, how can we get the phone back?