I still hope some days that I would be able to open my parent's minds and make them nicer people. It's a tough situation. I'm sorry you are stuck being back someplace that makes you feel so uncomfortable. I moved across the country to go to school when I was 17. I felt it was the best thing ever. The summers home were torture however. I suffered through them as best I could, and eventually stopped coming home when I got my own apartment out there. I would hazard a guess that my dad is a lot like your parents, and my mom is falling in that direction. I recently moved "home" again after 17 years of being away. I'm finding similar difficulties dealing with my mom, and wondering how I put up with her attitude and beliefs for so long as a child.
I try to get out of the house as much as I can, or occupy myself with things other than conversation when I am home. It's a bit tricky because my mom is pretty homophobic, and I moved back here with my wife... She likes us as individuals, but she still spouts some pretty mean things once in a while. It makes for an uncomfortable situation most of the time, so we try not to address her beliefs about us.
Can you try to keep the talking/interaction on levels that interest both of you? or at least on topics that don't drive you nuts? My mom is a science and tech geek, so we try to talk about that kind of stuff as opposed to politics and human rights. If she goes on one of her rampages, my wife and I will either "hide" in the bedroom, or leave the house. We can't change her mind on things (at least not overnight), so we make do with staying away as best we can...
I think one of the greatest lessons kids can learn from their parents is what not to do... It def does make it harder to come back home though...
((hugs)) it's almost time to go back to school. You can do this. (venting generally helps me, so if you want to vent away about them, please do so)