I wish I could answer it. :( I'm 18 too, and I'm kinda struggling with the same problem. Either way, I think being responsible and being mature is the best way out. Just keep good relationships with them. DoNOT argue with them. I know its hard, but you can punch a pillow later on, and scream. Take loads of initiatives and complete every task you've taken on yourself. It's retarded, I know. :(
But try it out for like three weeks, and do well in all your tests. Ooh, one tip I would suggest (this is for only when you're super exhausted with studying) would be just to act like you're studying any time they're around. :P The moment they walk into the room, act like you were getting your textbook, or just keep staring at the textbook until they go away. :P They tend to think you're super studying! Hehe, I'm no smartass but I'd like to think I am.
Hope this helps! :)