do you have other options for schools that you could go to instead?
get in touch with them with your GCSE grades (check their website to make sure you meet their entry requirements) and explain your situation and say that you believe you can pass the subject and you're happy to start from year 12 if they'll let you start in September.
you could also speak to your teachers at your current school, if you have a good reason for doing poorly in these exams maybe they'll let you retake them.