I'm 18 years old and my girlfriend is 15. We been together for a while now and her mom let us be together. But after time she started doing bad in school and thinks I'm her distraction . She doesn't want us to be together not even talking to each other. What can I do we still text without her knowing but I just want things back to normal. Her mom hates me now

And when she finds the girl texting most likely she loses phone and it gets worst.

At 18 you are considered a adult, start acting like one, if mom says no respect the mom and wait.
Girl proves she can do better in school

You haven't learned yet that you don't 'make' people do things (unless you have a gun)?
It's a sign of immaturity to say you want to. And this is not your concern at all; it's hers. Obviously she has to get her grades back up. Her whole life could be ruined by flunking high school. What are you doing with your adulthood? Are you a senior or out of school?

As an adult you can be charged with statutory rape if you had sex, or some variation on endangering a minor, just for being with her without parental permission.

You can't make her mom do anything, but you can act like a mature 18 year old and respect her wishes and leave this girl alone. Let her prove to her mom she can be trusted and bring her grades back up, and you prove you are decent and responsible.