Can't find time to have sex?

I can't find time to have sex with my girlfriend, we are both in highschool (none of us can drive yet though) and were trying to have sex but we cant because she has a large family, so someones always home, and her mom always ensures someone else is in the room with us. So we try to have sex at my house but every 10 minutes my mom or dad will come up and check on us. Can someone please help us? We've been trying to screw since November but we've never been able to be alone.