If you've already started and you only have a year left I really would carry on. I just finished sixth form I nearly left after I finished year 12. My head of Year made me come into school to talk about it and I didn't end up going to college. It was probably one of the best decisions I made. Otherwise I would have gone to college and had to restart all of my courses and then basically do an extra year for no reason. If you really don't want to be there because your grades are looking a bit crazy just do proper research before you decide to leave. You don't want to leave and then regret it later on and not be able to go back.