How do I get my girl friends parents to like me again?

Ok well me and this girl went out for almost a year, we fell short by 2 weeks for a whole year. Things ended very bad yelling, fighting crying, adn all that. We just stopped talking to each other for about 3 months and then it was christmas eve and I decided to talk to her. We started talking again and decided to go to the movies with some friends. Well when we got back and dropped her off. Her mom said I dont want you talking to cody anymore and I dont want you guys to hang out. So we decided to just sneak out at late nights to see each other. Everything was working out it was perfect she started to like me more and then about a weeks ago we had sex for the very very first time. It was the first time for the both of us and then we did it again 2 days later. Then we came up with the idea of after school her just telling her mom that she was hanging with someone and she just comes to my house and then I drop her off at the basketball game later. Well after school her mom went to school to spy on her, and once she got into my car her mom pulled up. So she got out of the car and went to her moms and left. Her mom took away her phone and grounded her. So now we really need to sneak even more. But she trys to talk to her parents all the time but they are way to stubborn and wont listen to her. She is17 and I am 18 so im graduating this year and she has another year to go. So we wont get to see each other at school so we wont get to see each other much anymore. So what can I do to try and get her parents to like me or at least let us hang out?