Do the schools decide on the entry requirements for their sixth form? Are they allowed to accept a student who got a grade lower than the requirement?
It's up to the school/sixth form in question. You should get in touch with them ASAP as most sixth forms have kicked off their academic years already.
Also, a word of warning: consider your other options as well. If you struggle to meet the GCSE requirement for the courses you want to do, A-levels are likely to be an uphill struggle, although not an impossible one, I'll admit.