Foreigner in Year 11 - GCSE

I'm a fifteen year old who just got in England (will be here for two years). I've been searching about the school system over here, and found out that we take exams (GCSEs) in the end of Year 11, which is the one I am supposed to go right now. Notwithstanding, the school council told me I was able to apply for Year 10 (which would get me two years to study for GCSEs) or Year 11 (which would give me the opportunity to get my GCSEs, do Year 12 later, get some my A-levels AND MAYBE IN THIS CASE ONLY finish Year 13 - I would try to live in another person's house and talk to the UK Embassy to expand my period over here). I'm confused: I do not have any idea about which one of the options I'll go with. Which would you recommend? Can I get good GCSE grades with only one year of study?

OBS: I am a higher than average student from Brazil (I've studied in great private schools over there) and I've lived in the USA (that's how I learned English).