Somebody sent something digusting on my phone to my friend what do i do?

Hi Lemme start from the very beginning: Me and one of my schoolmates were looking at my phone because i was texting ppl and i told this one girl that the schoolmate was saying stuff about her and such. Later when his older brother comes out he wants to see my phone and then we swap phones. While i'm playing the app on his phone he is texting some people on my list and then 15 minutes later i ask for my phone back to see if he sent anything to anyone and i find out he sent a disgusting text message to one of my friends and a gross picture. ( no i won't go into details on what that picture was). I then receive a text message from the girl who he sent the text and pic too and she starts swearing at me calling me a f** and a disgusting b****. I tell her it wasn't me but she doesn't believe me. I tell her it was her friends older brother and not me but then she doesn't believe me. Later this week i find out she texted alot of ppl telling them what had happened and saying it was me who did it. I had to tell everyone i didn't do it but people still wouldn't believe me. Even now as i am writing this i am still being accused of sending this nasty text and pic to her. I swear to god i feel like punching somebody so hard in the face. What do i do to help prove to everyone that i didn't send that message and to prove that it was the other guy?? ( BTW i was forced to say that it wasn't the older brother to the girl or else i would be beaten up even though i know it was him). What do i do to prove myself innocent?