What can i do for my mom to trust my bf and i again?

Well..My mom and i had a fight about me being disrespectful to her and all. She took my phone and read all my messages. Today i apologized to her about what i did yesterday, which was disrespecting her. And today she told me that she found out that my bf and i have had . She's threatening me that she will tell my dad, and he's a retired U.S marine, all strict and all. I'm my Boyfriends first gf, and he will do anything to gain her trust back, even if he has to talk to her on the phone and anything. He promised me that we werent going to do it again, because it just wasnt right. She told me about my reputation in the school, about how the girls will start calling me skank, slut, and yadayada. i dont really care. She said that she may never trust him again, what can him and i do to gain her trust back?